Juan Martin del Potro took Roger Federer's Swiss Indoors title on Sunday, beating the hometown favorite 6-4, 6-7 (5), 7-6 (3) in the final.
Del Potro showed an impressive all-around game to help avenge a heartbreaking loss to Federer in the Olympics Games semifinals in August.
"It's amazing for me," Del Potro, who never dropped serve, said in an on-court interview. "Always, playing against Roger is a pleasure. We know he's the best player ever in this sport."
Federer, who won the event in five of the past six years, was seeking his 77th career title to tie John McEnroe at fourth on the Open Era list.
Del Potro ended a six-match losing streak against the top-ranked Swiss this year, which also included a five-set defeat at the French Open.
"You deserved it," Federer told his opponent on court. "You had a couple of tough losses against me this year and I'm really happy for you."
The 24-year-old Argentinian won for the second straight week, after taking the Vienna title. His fourth title of the season is the 13th of his career.
Del Potro's victory in 2 hours, 44 minutes compared to his 4-hour, 26-minute defeat at the Olympics —the longest three-set match in history — when Federer won the decider 19-17 on Centre Court, Wimbledon.
As he did at Roland Garros and the Olympics, Del Potro took the first set, breaking Federer's serve in the fifth game.
Federer struggled briefly with his ground strokes, netting two forehands and a backhand to give Del Potro the lead.
The second-seeded Del Potro relied on solid serving and unleashed blistering forehands when needed. He took his first set point chance when Federer sent a backhand wide.
Both players wasted their only break chances in the second set, which hinged on a tense Federer service game at 5-5.
Facing break point, Federer struck a forehand crosscourt winner then saw Del Potro slip up by netting a sliced backhand on the next point.
Federer controlled the tiebreaker and punched the air when he leveled the match, as Del Potro let a backhand drift wide.
Federer held three break points at 1-1 in the deciding set, but Del Potro saved them before firing an ace to win the game.
A wild Federer forehand let another break chance slip in the seventh game, which Del Potro clinched with another ace.
Federer then struck three straight aces when threatened at 5-6 down to force another tiebreaker.
Del Potro seized his chance after Federer netted a topspin forehand, and clinched on his first match-point chance on another wayward forehand from the top-seeded Swiss.
Federer later pulled out of the Paris Masters to rest before the ATP World Tour Finals in London.
The top-ranked Swiss announced his move to withdraw from the tournament, which starts Monday.
"I'm not going. It's the only right decision for me," Federer said. "I had some niggling stuff and I didn't want to take a chance obviously before the World Tour Finals. I need some time to prepare."
Federer won the Paris trophy last year, and will defend his London title the following week.
Federer will start a record 302nd week ranked No. 1 on Monday, but risks losing that status to Novak Djokovic in the final two weeks of the season.