Jack Sock won the ATP Finals shootout with Alexander Zverev, beating the German in three sets to secure a semi-final berth.
With the unbeaten Roger Federer having sewn up top spot in this group, the equation for both was simple: win and you're through.
The American pulled off a stunning late-season surge to reach the London extravaganza and he has kept up his form with victories over Marin Cilic, and now Zverev 6-4 1-6 6-4.
Crucial momentum appeared to have swung Zverev's way when he recovered from a first set of multiple missed opportunities to wipe the floor with Sock in setting up a decider.
But the 20-year-old's sudden loss of composure when it mattered most cost him and Sock can now look forward to a final-four meeting with Grigor Dimitrov, who the American holds a 3-1 win-loss record over.
Sock began the contest agitated by persistent noises from the crowd, but he overcame the distraction to rescue two break points with a nice drop volley at the net and he had the breakthrough in game seven as a pair of crushing forehands were too good for the German.
The American faced three break points in the very next game, but dug his way out again as Zverev began to grow visibly frustrated - another drop shot caught the 20-year-old deep in the court to seal the first set.
Sock's joy was soon cut short, though, as Zverev completely ran away with the second set, charging into a 5-0 lead thanks in part to a fourth game which saw Sock send a 'tweener lob long before bunting a backhand into the tramlines.
Sock held game six, to preserve first serve in the decider, but he was immediately broken as Zverev came up big on defence, the American's anger getting the better of him as he walloped a ball into the crowd.
Though he was given a point penalty, Sock was soon smiling again as a Zverev double break and tame netted forehand coughed the initiative back up and the American had victory in sight when another double break put him 3-1 up.
There was still time for one last twist as two backhand errors put the game back on serve, but Zverev crumbled under the pressure of serving to stay in the contest, yet another double fault handing Sock match point before an error put an American in the last four for the first time since 2007.
Sock  bt Zverev  6-4 1-6 6-4
Sock – 24/37
Zverev – 17/30
Sock – 5/4
Zverev – 5/8
BREAK POINTS WON
Sock – 4/10
Zverev – 4/14
FIRST SERVE PERCENTAGE
Sock – 68
Zverev – 66
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Sock – 71/31
Zverev – 66/47
Sock – 84
Zverev – 87