Haas was ranked second in the world seven years ago
Germany's Tommy Haas beat world number four Novak Djokovic 6-3 6-7 (4-7) 6-1 to win the Halle grasscourt tournament.
It was the 31-year-old's first victory over the Serb, who was ranked number one for the competition, after French Open champion Roger Federer pulled out.
Haas raced through the first set, but double-faulted in the tie-break to lose the second. However, he broke Djokovic twice to win the decider with ease.
"I never believed that I would end the week with the title," said Haas.
"It is unbelievable. I would like to thank the public for their great support, it has really been a wonderful week for me."
It is his first ATP tournament victory since winning in Memphis in 2007 and proves that last year's persistent shoulder injury is behind him.
He won the opening set inside half an hour, but Djokovic held his serve throughout the second to take the set into a tie-break and took advantage of three double faults by Haas to level the match.
Haas, ranked 41 in the world, broke again to jump into a 2-0 lead in the decider, but Djokovic fought back to hold three break points in game three.
However, the German dug deep to hold his serve and although Djokovic won his next service game, Haas, raced through the next three to wrap up the win.
Djokovic, who only reached the final after an error-strewn victory over qualifier Olivier Rochus in the last four, conceded Haas had played the better tennis.
"Tommy completely deserved the win," he said.
"I did not play well at all and I was lucky to win the second set.
"I would have lost it without those double faults."
But the Serb, who had to save five match points in his second round game against Florent Serra earlier in the week, felt the week had been useful.
"Getting to the final is a good achievement," he said. "I had enough matches on grass before Wimbledon but I would not call myself one of the favourites though."