The stakes are beginning to rise in the third day of the 2020 ATP Finals at London's O2 Arena.
Having earned wins in their opening matches, Rafael Nadal and Dominic Thiem were looking to seal a place in the semifinals Tuesday.
Nadal needed one of two circumstances: a straight-set victory or a win and an Andrey Rublev win over Stefanos Tsitsipas. Thiem needed an upset of Nadal and Tsitsipas to knock off Rublev.
Survival is the focus for Tsitsipas and Rublev, who lost their openers. Dave Seminara of the ATP Tour's official site noted Kei Nishikora (2016) was the last player to advance after going 1-2 in the round-robin stage. David Nalbandian in 2006 was the last to move on after losing his first two matches.
Tuesday's second clash is in effect an ATP Finals eliminator.
No. 3 Dominic Thiem def. No. 2 Rafael Nadal 7-6 (7), 7-6 (4)
No. 6 Stefanos Tsitsipas def. No. 7 Andrey Rublev 6-1, 4-6, 7-6(6)
Full draw and standings are available on the ATP Finals' official site
For the second time in 2020, Thiem got the better of Nadal. Their Australian Open encounter featured three tiebreakers, and the margins in Thiem's win Tuesday were equally slim.
Neither player budged on serve in the first set. Thiem got the closest, pushing Nadal to deuce in the sixth game before the 20-time Grand Slam champion held.
Nadal looked to be in the driver's seat in the tiebreaker after jumping ahead 5-2. Thiem took the next three points to tie things up. Nadal still had two more opportunities to claim the set, with the 27-year-old Austrian battling back each time.
Up 8-7, Thiem unleashed a powerful cross-court forehand winner.
The second set played out largely as the first did. Nadal went up a break to take a 4-3 lead, but Thiem responded in kind in the next game.
Up 5-4, Thiem had three match points with Nadal on serve. The Spaniard dug deep to stay alive.
The 2020 U.S. Open champion wasn't to be denied in the end. He gained the upper hand in the second tiebreaker, and a backhand from Nadal sailed wide to bring the proceedings to a close.
Thiem did what he needed to qualify for the semifinals, and Tsitsipas did the rest.
The defending champion made quick work of Rublev in the first set, but the 23-year-old Russian composed himself during the brief interlude between sets.
Having dropped serve twice in the opening frame Rublev won all but one of his 18 first-service points in the second stanza, per the ATP Finals' official site. Tsitsipas had two break-point chances in the seventh game but couldn't capitalize. Had he converted on either one, the match probably would've ended in straight sets.
Instead, Rublev held in the ninth game to go up 5-4 and swept the next game to make it one set apiece. He was again up to the challenge with the score deadlocked at one in the third set, fending off five break chances for Tsitsipas.
Rublev had two double faults over the course of the evening. The first came in the second set and the second arrived at the most inopportune time possible. Serving for the match, he missed the mark with his second serve to hand Tsitsipas a lifeline.
The 22-year-old didn't look a gift horse in the mouth and nabbed the next two points for the victory.
Rublev is left to rue his double fault and how that could be the mistake that seals his fate in the group stage.