Stefanos Tsitsipas collects 1,300 ATP Rankings points for his Nitto ATP Finals triumph. © Julian Finney/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut are competing at the Nitto ATP Finals for the fifth straight year. © AFP/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut are the first French team to lift the Nitto ATP Finals trophy since 2005. © Peter Staples/ATP Tour

Nicolas Mahut and Pierre-Hugues Herbert are aiming to become the first French team since 2005 to lift the Nitto ATP Finals trophy. © AFP/Getty Images

Stefanos Tsitsipas is through to his sixth final of the year (2-3). © Peter Staples/ATP Tour

Juan Sebastian Cabal and Robert Farah end their 2019 season with five team titles and a 50-19 record. © AFP/Getty Images

Nicolas Mahut and Pierre-Hugues Herbert finish Group Max Mirnyi with a 3-0 record. © AFP/Getty Images

Rafael Nadal improves to 5-1 in his FedEx ATP Head2Head series against Stefanos Tsitsipas. © Peter Staples/ATP Tour

Rafael Nadal joins Roger Federer, Novak Djokovic and Jimmy Connors with five ATP Tour year-end No. 1 finishes. © AFP/Getty Images

Roger Federer breaks Novak Djokovic on three occasions to reach the final four at the Nitto ATP Finals on Thursday. © Peter Staples/ATPTour

Filip Polasek and Ivan Dodig improve to 3-0 in their FedEx ATP Head2Head series against Raven Klaasen and Michael Venus. © James Chance/Getty Images

Filip Polasek and Ivan Dodig end their 2019 season with a 27-11 team record. © Peter Staples/ATP Tour

Matteo Berrettini becomes the first Italian to win a singles match at the Nitto ATP Finals. © Peter Staples/ATP Tour

Stefanos Tsitsipas defeats Alexander Zverev and reaches the semi-finals at the Nitto ATP Finals. © Peter Staples/ATP Tour

Pierre-Hugues Herbert and Nicolas Mahut reach the last four at the Nitto ATP Finals for the second straight year. © Julian Finney/Getty Images

Nicolas Mahut and Pierre-Hugues Herbert own a 16-5 record as a team this year. © Peter Staples/ATP Tour

Jean-Julien Rojer and Horia Tecau prevailed at the Nitto ATP Finals in 2015. © Peter Staples/ATP Tour

Dominic Thiem scores an epic third-set tie-break win against Novak Djokovic to reach the semi-finals. © Peter Staples/ATP Tour

Joe Salisbury and Rajeev Ram are still in contention to advance out of round-robin action at the Nitto ATP Finals. © Peter Staples/ATP Tour

Stefanos Tsitsipas secured his first win in six tries against Daniil Medvedev at the Nitto ATP Finals. © Peter Staples/ATP Tour

Andreas Mies and Kevin Krawietz pick up their maiden win at the Nitto ATP Finals. © Peter Staples/ATP Tour

Marcelo Melo and Lukasz Kubot level their FedEx ATP Head2Head series against Ivan Dodig and Filip Polasek at 2-2. © Peter Staples/ATP Tour

Novak Djokovic is chasing a sixth year-end No. 1 finish in the ATP Rankings. © Julian Finney/Getty Images

Mike Bryan and Jack Sock battle hard on Sunday to work their way back into the doubles final at the Nitto ATP Finals. © Clive Brunskill/Getty Images

Prominent players from the 2000s are celebrated at the Finals Club on finals day at the Nitto ATP Finals on Sunday. © Peter Staples/ATP World Tour

Andy Ram, Gustavo Kuerten and Jonathan Erlich pose for a selfie at the Finals Club on Sunday. © Peter Staples/ATP World Tour

Jack Sock and Mike Bryan celebrate lifting the doubles crown at the Nitto ATP Finals. © Julian Finney/Getty Images

Alexander Zverev celebrates with his dog, Lovik, and parents after beating Novak Djokovic to win the Nitto ATP Finals on Sunday. © Clive Brunskill/Getty Images

Alexander Zverev is the youngest Nitto ATP Finals champion since 2008, when final opponent Novak Djokovic lifted his maiden title at the season-ending event in Shanghai. © 2018 Getty Images

Alexander Zverev becomes the first German champion at the Nitto ATP Finals since Boris Becker in 1995. © Julian Finney/Getty Images

Novak Djokovic falls short of tying Roger Federer's record of six Nitto ATP Finals titles. © Justin Setterfield/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut will meet Mike Bryan and Jack Sock in the championship match at the Nitto ATP Finals on Sunday. © Clive Brunskill/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut have lifted two tour-level trophies as a team this season. © Justin Setterfield/Getty Images

Jack Sock meets ball kid Jessica Matthews after their collision during his semi-final alongside Mike Bryan at the Nitto ATP Finals on Saturday.

Alexander Zverev defeats Roger Federer in straight sets at the Nitto ATP Finals on Saturday. © Julian Finney/Getty Images

Alexander Zverev becomes the first German since 1996 to reach the championship match at the Nitto ATP Finals. © Peter Staples/ATP World Tour

Jack Sock and Mike Bryan have captured two Grand Slam titles this year. © AFP/Getty Images

Novak Djokovic is pursuing his sixth title at the Nitto ATP Finals. © Julian Finney/Getty Images

Roger Federer falls short in his pursuit of a seventh Nitto ATP Finals title, losing against Alexander Zverev. © Peter Staples/ATP World Tour

Nicolas Mahut and Pierre-Hugues Herbert advance to the championship match at the Nitto ATP Finals for the first time. © Peter Staples/ATP World Tour

Alexander Zverev and Boris Becker are the last two Germans to reach the championship match at the Nitto ATP Finals. © ATP

Chief Player Officer Ross Hutchins presents recently-retired players Florian Mayer, Andre Sa and Gilles Muller with gifts at the Nitto ATP Finals on Friday. © Peter Staples/ATP World Tour

Lukasz Kubot watches on as Marcelo Melo strikes a drive volley forehand on Friday at the Nitto ATP Finals. © Julian Finney/Getty Images

Lukasz Kubot and Marcelo Melo defeat Oliver Marach and Mate Pavic in straight sets at the Nitto ATP Finals on Friday. © Peter Staples/ATP World Tour

Alexander Zverev has lifted three tour-level trophies this season. © Peter Staples/ATP World Tour

Novak Djokovic finishes round-robin play at the Nitto ATP Finals undefeated for the fifth time with a win against Marin Cilic. © Clive Brunskill/Getty Images

Novak Djokovic beats Marin Cilic in straight sets for the first time in four tries in 2018. © Peter Staples/ATP World Tour

Juan Sebastian Cabal and Robert Farah defeat Raven Klaasen and Michael Venus in straight sets at the Nitto ATP Finals on Thursday. © Peter Staples/ATP World Tour

Robert Farah watches Juan Sebastian Cabal smash on Thursday against Raven Klaasen and Michael Venus at the Nitto ATP Finals. © Peter Staples/ATP World Tour

Roger Federer defeats Kevin Anderson in straight sets at the Nitto ATP Finals on Thursday. © Peter Staples/ATP World Tour

Jamie Murray and Bruno Soares will contest their third straight semi-final at the Nitto ATP Finals on Saturday. © Alex Pantling/Getty Images

Bruno Soares and Jamie Murray defeat Henri Kontinen and John Peers in three sets at the Nitto ATP Finals on Thursday. © Peter Staples/ATP World Tour

Dominic Thiem is making his third straight appearance at the Nitto ATP Finals. © Peter StaplesATP World Tour

Mike Bryan and Jack Sock are appearing in just their eighth tour-level event as a team. © Julian Finney/Getty Images

Novak Djokovic is aiming to reach the semi-finals for the eighth time at the Nitto ATP Finals. © Julian Finney/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut are appearing at the Nitto ATP Finals for the fourth straight year. © AFP/Getty Images

Pierre-Hugues Herbert and Nicolas Mahut defeat Lukasz Kubot and Marcelo Melo in straight sets at the Nitto ATP Finals on Wednesday. © Peter Staples/ATP World Tour

Kevin Anderson defeats Kei Nishikori in straight sets at the Nitto ATP Finals on Tuesday. © Peter Staples/ATP World Tour

Robert Farah and Juan Sebastian Cabal will meet Raven Klaasen and Michael Venus for a place in the semi-finals at the Nitto ATP Finals on Thursday. © Peter Staples/ATP World Tour

Roger Federer owns a record six Nitto ATP Finals titles. © Clive Brunskill/Getty Images

Roger Federer defeats Dominic Thiem in straight sets at the Nitto ATP Finals on Tuesday. © Peter Staples/ATP World Tour

Jamie Murray and Bruno Soares defeat Juan Sebastian Cabal and Robert Farah in straight sets at the Nitto ATP Finals on Tuesday. © Peter Staples/ATP World Tour

Four-time participant Kei Nishikori battles hard to find a way into his match against Kevin Anderson on Tuesday at the Nitto ATP Finals. © Julian Finney/Getty Images

Kei Nishikori drops to 1-1 in Group Lleyton Hewitt after straight-sets loss to Kevin Anderson at the Nitto ATP Finals on Tuesday. © Peter Staples/ATP World Tour

Michael Venus and Raven Klaasen move to 1-1 in Group Llodra/Santoro after beating Nikola Mektic and Alexander Peya at the Nitto ATP Finals on Tuesday. © Naomi Baker/Getty Images

Novak Djokovic is aiming to lift his first Nitto ATP Finals trophy since 2015. © Peter Staples/ATP World Tour

Novak Djokovic takes a selfie with fans after defeating John isner in straight sets at the Nitto ATP Finals on Monday. © Peter Staples/ATP World Tour

Novak Djokovic extends his FedEx ATP Head2Head series lead against John Isner to 9-2 on Monday in London. © Peter Staples/ATP World Tour

Mate Pavic and Oliver Marach defeat Pierre-Hugues Herbert and Nicolas Mahut in straight sets at the Nitto ATP Finals on Monday. © Peter Staples/ATP World Tour

Alexander Zverev is making his second straight appearance at the Nitto ATP Finals. © Clive Brunskill/Getty Images

Cristiano Ronaldo is thrust into action during Novak Djokovic's victory against John Isner on Monday at The O2. © AFP/Getty Images

Kevin Anderson improves to 7-2 in his FedEx ATP Head2Head series against Dominic Thiem after his straight-sets win at the Nitto ATP Finals on Sunday. © Peter Staples/ATP World Tour

Juan Sebastian Cabal and Robert Farah, in their eighth full season as a team, are making their debut at the Nitto ATP Finals. © Clive Brunskill/Getty Images

Jamie Murray and Bruno Soares defeat Raven Klaasen and Michael Venus in three sets at the Nitto ATP World Tour Finals on Sunday. © Clive Brunskill/Getty Images

Kei Nishikori defeats Roger Federer in straight sets at the Nitto ATP Finals on Sunday. © Clive Brunskill/Getty Images

Kei Nishikori improves to 43-19 at tour-level this season after beating Roger Federer at the Nitto ATP Finals on Sunday. © Peter Staples/ATP World Tour

The eight singles players competing at the Nitto ATP Finals come together at London's iconic Houses of Parliament for the Nitto ATP Finals Official Launch presented by Moët & Chandon on November 9, 2018 in London, England. The world’s biggest indoor tennis tournament will be played 11-18 November at The O2. © Clive Brunskill/Getty Images

Kevin Anderson and his wife, Kelsey, take a boat ride to The O2 on Thursday ahead of the Nitto ATP Finals. © ATP

First-time qualifier Kevin Anderson trains on Thursday with Novak Djokovic ahead of the Nitto ATP Finals. © ATP

Novak Djokovic trains ahead of the Nitto ATP Finals at The O2 with Kevin Anderson on Thursday. © ATP

Six-time former champion Roger Federer, with coach Ivan Ljubicic looking on, practises on Thursday ahead of the Nitto ATP Finals. © ATP

© ATP

After a two-hour, 30-minute battle against David Goffin, Grigor Dimitrov emerged with the Nitto ATP Finals trophy in his debut at The O2. © Thomas Lovelock/ATP World Tour

Grigor Dimitrov finishes his debut at The O2 as an undefeated champion, beating David Goffin for the title. © Thomas Lovelock/ATP World Tour

A year after debuting at The O2 as an alternate, David Goffin attempts to claim the Nitto ATP Finals trophy. © Peter Staples/ATP World Tour

© Peter Staples/ATP World Tour

David Goffin serves during his victory against Roger Federer at the Nitto ATP Finals on Saturday. © Thomas Lovelock/ATP World Tour

Jack Sock waves to the crowd at The O2 after losing in the semi-finals on Saturday. © Thomas Lovelock/ATP World Tour

David Goffin strikes a backhand volley during his round-robin match against Dominic Thiem on Friday. © Peter Staples/ATP World Tour

Roger Federer strikes a backhand during his win against Marin Cilic on Thursday at The O2. © Thomas Lovelock/ATP World Tour

Ryan Harrison hits a forehand during his match with Michael Venus against Jean-Julien Rojer and Horia Tecau on Thursday. © Peter Staples/ATP World Tour

Grigor Dimitrov cruised past David Goffin on Tuesday to secure a spot in the semi-finals at the Nitto ATP Finals. © Thomas Lovelock

Dominic Thiem strikes a forehand during his first victory of the 2017 Nitto ATP Finals at The O2 in London on Wednesday. © Thomas Lovelock/ATP World Tour

United States' Jack Sock defeated Croatia's Marin Cilic on Tuesday for his first win at the Nitto ATP Finals in London. © Thomas Lovelock