Marin Cilic fought back to keep alive his hopes of reaching the semi-finals of the ATP Finals for the first time with victory over John Isner.
The Croat, who has won just two of 11 matches in four appearances at the event, triumphed 6-7 (2-7) 6-3 6-4.
The result puts Novak Djokovic into the semi-finals, leaving Cilic, Isner and Alexander Zverev to fight it out for the group's other spot on Friday.
It was the first singles match of this year's event to go to a deciding set.
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Cilic holds his nerve
Cilic has something of a reputation as a bottler - the most recent example coming last week in the quarter-finals of the Paris Masters where he lost to Djokovic despite having won the first set.
And there was a moment here when it looked like he might not hold his nerve.
Having secured rare back-to-back breaks against the tour's leading server to win the second set and take an early advantage in the third, he then inexplicably dropped serve to give Isner hope.
But he then showed his determination - via some first-class returning and a wonderful drop volley - to secure a third successive break.
With Isner holding on to force him to serve for the match, Cilic showed no sign of wobbling as he brought up three match points and took victory when the American returned a looping second serve wide.
"It was very difficult. The first set was extremely tough. John played a great tie-break and I had to stay focused," said Cilic, whose only other Finals victory came against Kei Nishikori in 2016.
"I served really well and I am really pleased with the win. I felt that I had that momentum towards the end of the second set. I felt the ball well and got some good points on Isner's serves.
"He hit a few double-faults on the break points - that helped - but I stayed patient and I managed to break back and play some good tennis towards the end."
Isner faces Zverev on Friday, while Cilic takes on the already-qualified world number one Djokovic with one other semi-final place at stake.
The other group is back in action on Thursday for their final round-robin matches, with Roger Federer against Kevin Anderson after Dominic Thiem takes on Kei Nishikori. All four in that group have a chance of progressing.
|Group Gustavo Kuerten|
Isner let down by double faults
Isner was playing after learning of the death of one of his close friends and former strength coach Kyle Morgan, and he wore the initials KM on his shoes.
Morgan. 35, was found dead on Monday after being reported missing by his family.
"I found out two nights ago after I lost to Novak (Djokovic)," Isner said. "It's difficult, very difficult.
"When I left college, I worked out exclusively with Kyle for probably seven years. That's four, five times a week for seven years. We lived together, as well.
"We were certainly very close. It's devastating not just for me, but for a whole bunch of people. He was a fantastic guy."
As the leading server on the Tour, it was no surprise that Isner began the match with a thumping 138mph ace.
He delivered a total of 17 aces against Cilic but was let down by six double faults, including the one that handed the Croat the crucial break in the second set.
The American is making his debut at the tournament after enjoying his best year to date, having reached his maiden Grand Slam semi-final at Wimbledon, won his first Masters 1000 title at Miami and finished the season ranked inside the world's top 10 for the first time.