Alexander Zverev produced a blistering finish to win a back-and-forth encounter 6-4 3-6 6-4 against Marin Cilic on his ATP Finals debut.
Much has been expected of the third seed in London after a season that has yielded five titles in a breakout 2017.
A straightforward victory appeared in the German's grasp after he took the first set against Cilic, who is back in England's capital for the first time since an emotional Wimbledon final loss to Roger Federer.
The tables were turned as Cilic dominated the second and led 3-1 in the decider, before Zverev won five of the last six games to earn a thrilling victory in the Group Boris Becker clash at the O2 Arena.
The early ascendancy was all with Finals debutant Zverev, who mixed up his game from the off and, after Cilic skewed wide at the end of a 29-shot rally, the German broke with an excellent drop shot at the net.
A hold to love for a 2-0 lead suggested that Cilic would find the going difficult, but Zverev had to save a break point with a huge ace in game four before surrendering a 40-0 lead in the sixth, only to battle to retain the advantage.
Cilic rebounded from those missed chances to quickly gain the upper hand in the second set, and he had a 2-0 lead when Zverev, having saved two break points, mishit a forehand.
The Croatian's big hitting was now causing his opponent plenty of problems, and Zverev saving set point with an overhead smash in game eight only slightly held up Cilic.
The momentum was truly in Cilic's favour when he earned the first break of the decider, although a couple of poor errors from Zverev assisted him.
But to his credit, Zverev stuck to his task and, when presented with his first break-point chance of the third, he produced a sensational lob from the left that Cilic framed wide.
And the turnaround was complete when Cilic, serving to stay in the match at 5-4 down, was broken to love – Zverev rounding out the win with a volley at the net after giving his opponent the runaround.
Zverev  bt Cilic  6-4 3-6 6-4
Zverev – 31/41
Cilic – 20/31
Zverev – 9/2
Cilic – 5/1
BREAK POINTS WON
Zverev – 3/4
Cilic – 2/6
FIRST SERVE PERCENTAGE
Zverev – 55
Cilic – 63
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Zverev – 81/48
Cilic – 74/57
Zverev – 88
Cilic – 86