Austria's Dominic Thiem maintained his hopes of reaching the ATP Finals last four with a straight-set win over Japan's Kei Nishikori in London.
The world number eight, who lost to Kevin Anderson and Roger Federer in straight sets in his previous group matches, won 6-1 6-4.
It means Federer will advance to the semi-finals if he wins a set against Anderson at 20:00 GMT on Thursday.
But if Anderson wins in two sets Thiem could progress with the South African.
Thiem has been a regular fixture in the world's top 10 over the past couple of years and, after reaching his first Grand Slam final at the French Open in June, came to the season-ending tournament looking to prove his credentials on a bigger stage once again.
But, before meeting Nishikori, he knew he faced an uphill battle to reach the semi-finals at the O2 Arena for the first time.
After performing well below par in his defeats against Anderson and Federer, Thiem upped his first-serve percentage to put pressure on Nishikori who, like in his loss to Anderson, made a significant number of unforced errors.
The second set was tighter as Nishikori rallied but another break for Thiem put him in command with a 4-3 lead.
Superb court coverage from the Austrian kept him in the point at 30-0 in what proved to be the final game, a brilliant forehand winner bringing up three match points.
And he needed only one, thumping down an ace out wide to seal victory.
"Whatever happens in the night match it is good to get a win here," Thiem said.
"I wasn't thinking about winning in two sets, I wasn't playing good in my other matches, so I just wanted to show my real self and it was way better."
Unlike Thiem, Nishikori now knows he has no chance of reaching the semi-finals - no matter what happens between Anderson and Federer.
Since opening the tournament with a shock 7-6 (7-4) 6-3 win over Federer, the world number nine has looked a shadow of his usual self.
He managed to claim only another six games in his matches against Anderson and Thiem combined and, against the Austrian, made 42 unforced errors.
An indication of his troubles came when the usually serene 29-year-old smashed his racquet on the ground during a second set in which he failed to earn a single break point.
Nishikori qualified for the finals after a strong end to the season