Jack Sock mounted a spirited fightback to claim a first ATP World Tour Finals victory that leaves Marin Cilic facing an early exit.
Sock defied the odds to qualify for the season-ending tournament for the first time and boosted his chances of reaching the semi-finals with a 5-7 6-2 7-6 (7-4) victory over the fifth seed in Group Boris Becker.
Both players could ill afford another defeat after losing their first matches at the O2 Arena and there was a sense of deja vu for Cilic, who led Alexander Zverev 3-1 in the final set of his opening contest before being consigned to defeat.
The Croatian led Sock by the same margin in the decisive set on Tuesday and was 4-2 up in the tie-break, but the eighth seed from America refused to be beaten and has now won all three matches against Cilic.
Cilic faces Roger Federer in his final round-robin match, while Sock will be up against Zverev in London as he attempts to pull off more stunning exploits in what has been a glorious end to the year.
Sock struck an early blow with a break in the opening game when Cilic netted a forehand, but the world number five hit straight back to level at 1-1.
Both players were solid on serve until the 2014 US Open champion went 0-40 up when leading 6-5 and sealed the set when Sock stooped to drill a forehand low into the net.
Sock blasted a ball into the crowd in frustration, but channeled his anger to break in the first game of the second set and was clearly enjoying himself, moving around the court nimbly as he levelled the match in dominant fashion.
Cilic appeared to have left plenty in the tank as he bossed the start of the final set, earning his rewards for an aggressive approach to open up a 3-0 lead.
Sock showed the sort of fighting spirit that enabled him to win the Paris Masters title and sneak into the last Finals spot, getting back on serve when Cilic fired a forehand into the tramlines under pressure.
Cilic looked set to come out on top when he claimed the first mini-break of the tie-break, but Sock showed incredible athleticism to dart in and guide away a winner after a Cilic shot struck the net cord and he won five points in a row to seal a hard-fought victory.
Sock  bt Cilic  5-7 6-2 7-6 (7-4)
Sock – 33/35
Cilic – 35/37
Sock – 9/2
Cilic – 13/4
BREAK POINTS WON
Sock – 4/10
Cilic – 3/7
FIRST SERVE PERCENTAGE
Sock – 59
Cilic – 63
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Sock – 75/53
Cilic – 77/49
Sock – 103
Cilic – 102