Alexander Zverev produced a majestic performance to claim the biggest title of his career and deny Novak Djokovic a record-equalling ATP Finals success on Sunday.
Djokovic had not been broken in London or dropped a set this week, but the magnificent Zverev struck a big blow for the next generation by winning a high-quality final 6-4 6-3.
World number one Djokovic was unable to match Roger Federer's tally of six triumphs at the season-ending event as his 21-year-old opponent rose to the occasion with a nerveless display.
The German was booed after stopping mid-point in a second-set tie-break when a ballboy dropped a ball during his semi-final win over Federer on Saturday, but he turned jeers into cheers by denying Djokovic a glorious end to a resurgent season.
Zverev, comfortably beaten by Djokovic in the round robin stage on Wednesday, won 79 per cent of points on his first serve as he claimed a fourth title of a season in which he has won more matches than any other player.
The third seed showed no signs of nerves on the big stage as both players started with vigour, serving superbly and trading punches from the baseline.
There was not a sniff of a break point in a service masterclass until Djokovic let his usual high standards slip at 4-4, drilling a forehand long and crashing another into the net to give Zverev the chance to serve out the set.
The world number five grasped his opportunity emphatically, firing down three aces and clenching his fist after another wayward groundstroke saw Djokovic lose a first set of the week.
Zverev maintained his momentum with the 14-time grand slam champion looking vulnerable and broke in the first game of the second set with a searing forehand winner to end an epic rally.
The Serbian broke straight back, but gifted his youthful opponent the initiative immediately with a poor attempted drop shot followed by a tired looking forehand into the net.
Zverev consolidated the break on this occasion to lead 3-1 and Djokovic was unable to find a way back.
A fittingly sublime backhand winner from Zverev ended a pulsating contest, the two rivals embracing after the new champion had dropped to the court with a weight seemingly lifted off his shoulders.
Zverev  bt Djokovic  6-4 6-3
Zverev - 23/18
Djokovic - 7/20
Zverev - 10/3
Djokovic - 1/1
BREAK POINTS WON
Djokovic - 1/1
FIRST SERVE PERCENTAGE
Djokovic - 60
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Zverev - 79/54
Djokovic - 74/35
Zverev - 57
Djokovic - 46
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