Tomas Berdych secured his first win at the 2011 ATP World Tour Finals with a hard-fought 2-6 6-3 7-6 (8-6) victory over Janko Tipsarevic.
Tipsarevic, who is the beneficiary of Andy Murray's withdrawal, won the first set at a canter.
Berdych then broke the Serb in the eighth game of the second set to force his way back into the match.
Tipsarevic briefly held match point in the tie-break but a subsequent double fault let the Czech in for victory.
It was Berdych's second victory inside a month over Tipsarevic after beating the 27-year-old en route to the semi-finals of the Paris Masters.
The world number seven's good form had carried over into his opening match at the O2 Arena in which he squandered match point in defeat by world number one Novak Djokovic.
"I was a bit unlucky on Monday against Djokovic but I got luckier here so there is still a good chance on Friday to qualify," said Berdych after his win.
"Before the tie-break I was feeling pretty good, I would say pretty confident. I did not think about what was happening the last time against Djokovic."
It was Tipsarevic, who had less than 24 hours' notice that he would be taking Murray's place in the event, who began quicker.
He dropped only six points on his serve as he strolled to the opening set with Berdych looking far from comfortable against an opponent who had won four of their previous five meetings.
Berdych's misfiring serve began functioning in the second set and he secured its only break in the eighth game before holding his nerve to serve out in the next game.
The pair could not be split down the back straight as Berdych held out under occasional pressure from Tipsarevic before a dramatic tie-break.
From 5-4 up, Berdych committed two errors to gift Tipsarevic match point.
But the contest swung back the Czech's way as Tipsarevic sprayed wide and then fluffed his serve.
Tipsarevic ended the contest on the court floor after slipping in pursuit of Berdych's final shot and limped to the net to share a handshake with the victor.
Spain's Nicolas Almagro would be the next player in line to join the tournament should Tipsarevic be unable to fulfil his final group match against world number one Novak Djokovic.
In Group A of the doubles event fourth seeds Mahesh Bhupathi and Leander Paes beat seventh seeds Jurgen Melzer and Philipp Petzschner 7-5 6-3.
It was the Indian pair's first group win, after their opening defeat to Robert Lindstedt and Horia Tecau, while Melzer and Petzschner suffered their second straight defeat of the group stage.