Alexander Zverev stunned world number one Novak Djokovic in straight sets to win the season-ending ATP Finals and earn the biggest victory of his career.

Djokovic, 31, was the heavy favourite to win a sixth finals title but was outclassed 6-4 6-3 by the German.

Zverev, the third seed, broke Djokovic for a 5-4 lead in the first set, then served three big aces to take the set.

The 21-year-old continued to dominate in the second and sealed a memorable victory as Djokovic faltered.

"I'm unbelievably happy. Obviously it is the biggest title I have ever won," said Zverev, who took home about £2m in prize money.

"How I played today, how I won it, for me it's just amazing."

He had lost to Djokovic in straight sets in the round-robin stage but, after beating Roger Federer in Saturday's semi-finals, produced a more confident performance to stop the Serb equalling Federer's haul of end-of-season titles.

Zverev missed his first championship point before hitting a sensational backhand down the line for victory, falling to the court in a mixture of disbelief and celebration as Djokovic jumped over the net to congratulate him.

"You definitely played much better than you did the group stage," Djokovic told him with a laugh. "You deserved to win."