London - Alexander Zverev last year denied Novak Djokovic a record-equalling sixth title at the ATP Finals, but the Serbian is the man to beat at the O2 Arena in London this year.
A look at the eight-strong field for the tournament, starting on Sunday:
New world No 1 and top seed Rafael Nadal secured his customary French Open title in June before winning his fourth US Open crown in September. The 33-year-old Spaniard, who has never won the ATP Finals, missed the tournament last year through injury and there are again questions over his fitness as he seeks to finish the year in top spot.
Roger Federer has won four titles in 2019 but his season will be remembered for the one that got away at Wimbledon, where he held two championship points on his own serve before eventually succumbing to Djokovic. The 38-year-old Swiss player holds the record for the most titles at the end-of-season event but has not won since 2011 and could see his record equalled by the Serbian this year.
Russia's Daniil Medvedev, 23, has had a breakthrough year, winning four titles and reaching the US Open final, where he battled back from two sets down to square the match against Nadal only to lose the decider. His victory at the Shanghai Masters, where he beat Zverev in the final, was his second straight Masters title.
Austria's Dominic Thiem, 26, will be playing at the ATP Finals for a fourth consecutive year after an impressive season in which he has won five titles, including his first Masters crown at Indian Wells, where he beat Federer in the final. But Thiem has yet to progress past the round-robin phase at the end-of-season tournament.
Stefanos Tsitsipas has progressed from lifting the Next Gen ATP Finals trophy last year to qualifying for the ATP Finals for the first time. The 21-year-old, the first Greek player to break into the top five of the ATP rankings, has won two ATP Tour titles in 2019.
The 2019 season has been a relative disappointment for Germany's Alexander Zverev after his victory against Djokovic in the final of last year's ATP Finals. He crashed out in the first round at Wimbledon and failed to make it past the quarter-finals in any of the Grand Slams. The 22-year-old's win at the O2 Arena last year appeared to underline his status as the leader of the next generation but he has fallen behind Medvedev.
Matteo Berrettini clinched the final spot at the ATP Finals following results at the Paris Masters, becoming the first Italian to qualify in singles since Corrado Barazzutti in 1978. The 23-year-old, who was ranked 57th in the world in March, has soared up the rankings, winning two ATP titles.
Andre Agassi Group:
Nadal, Medvedev, Tsitsipas, Zverev
Bjorn Borg Group:
Djokovic, Federer, Thiem, Berrettini