With Dominic Thiem having already qualified for the semifinals, another berth could be locked in by Wednesday's end in the 2020 ATP Finals.
Novak Djokovic and Daniil Medvedev prevailed in their opening match of the group stage, potentially putting them one win away from advancing. Their semifinal permutations hinged on the outcome of Alexander Zverev's encounter with Diego Schwartzman.
Zverev and Schwartzman were playing just to stay alive, since an 0-2 start would all but doom their chances of moving on.
No. 7 Alexander Zverev def. No. 8 Diego Schwartzman 6-3, 4-6, 6-3
No. 4 Daniil Medvedev def. No. 1 Novak Djokovic 6-3 6-3
Full draw and standings are available on the ATP Finals' official site.
Medvedev pulled off the biggest stunner of the event so far, upsetting the five-time champion in straight sets.
Djokovic has ascended to a status where his opponents can sometimes be beaten before the match even starts. They know they're facing long odds, and that leads to a level of caution that makes the task toppling the best player in the world more difficult.
Medvedev, on the other hand, isn't fazed when Djokovic is standing on the other side of the court.
If anything, the shoe was on the other foot Wednesday from the seventh game of the match on.
The game extended for 18 points. Djokovic served with the advantage four times yet still couldn't close the deal to tie the set at four all. On his third break point, Medvedev got the better of Djokovic and went ahead 4-3.
Sometimes, Djokovic allows his performance to lull and needs a level of adversity to wake himself up. That seventh game was one such example of a moment when he'd change the course of a match after experiencing a setback.
The opposite transpired as Medvedev won six straight games across the first and second sets. By the time the run came to an end, he was up 3-0. Djokovic held serve for the remainder of the match, which wasn't enough to force a third set.
Now, Medvedev is in the semifinals after Zverev provided the necessary aid.
Zverev and Schwartzman split their four meetings before facing off in the 2020 ATP Finals. Zverev now has a 3-2 head-to-head edge thanks to his victory Wednesday.
The service game proved to be a decisive factor in the match.
Zverev finished with 10 aces to zero for Schwartzman, per the ATP Finals' official site. That helped him win 73 percent of his first-service points. The 23-year-old German also played only 22 points on his second serve, with Schwartzman having 12 more second-service points over the same number of games.
The Argentine star didn't make it easy for Zverev, though.
Having already fallen a set behind, he was on the ropes early in the second set. He failed to convert two break-point opportunities in the first game and dropped serve en route falling 1-3 behind.
Schwartzman didn't panic and instead claimed five of the next six games to level the match.
His comeback bid came undone when he was serving at 2-2 in the third set. Zverev put himself in the driver's seat with a break and had two more break points while up 4-2. Schwartzman held, which only prolonged the inevitable.
"It was much better than on Monday," Zverev said in his post-match interview, per the ATP Tour's official site. "Diego is a very difficult player. He deserves to be here. ... There are no easy matches, but I am happy to get the win. I am happy to give myself a chance to go to the semifinals and I am excited for Friday."