Jack Sock kept his dreams of a fairytale finish to his season alive on Tuesday, beating Wimbledon finalist Marin Cilic 5-7, 6-2, 7-6 (7/4) at the ATP Finals.
Cilic, who won the 2014 US Open, broke the American twice to win the first set but found himself adrift in the second set as Sock levelled the round robin match with two breaks of his own.
In a see-saw deciding set, the Croatian fifth seed was quickly out of his blocks, racing to a 3-0 lead before Sock, the eighth seed, steadied himself, cancelling out the break.
The match then settled back on serve and went into a tie-break and Sock emerged as the winner.
Sock, the American number one, came from nowhere to reach the end-of-season event, reaching London by winning the Paris Masters — and breaking a run of European success at Masters events stretching back to 2010.
Sock lost his first match in the Boris Becker group to Roger Federer while Alexander Zverev defeated Cilic, leaving both fighting for their lives on Tuesday.
Later, the crowd will be treated to the latest episode in the cross-generational rivalry of Federer and Zverev, with the tournament desperately needing a compelling story after the withdrawal of world number one Rafael Nadal.
The Spaniard quit the tournament on Monday after losing his opening match in the Pete Sampras group to David Goffin.
Federer, 36, and Zverev, 20, have won two games apiece against each other. The Swiss world number two beat the German in Halle but Zverev returned the favour at the Rogers Cup in Montreal.
The ATP Finals feature the top eight fit male singles players and doubles teams who have accrued the most points throughout the 2017 season.