Qualifier Federico Delbonis caused a major shock by defeating top seed Roger Federer 7-6 (7) 7-6 (4) to reach the final of the German Tennis Championships.
Federer, a 17-time Grand Slam champion and four-time winner in Hamburg, took a wild card for the Hamburg tournament after losing to world No 116 Sergiy Stakhovsk in the second round at Wimbledon.
The Swiss star, who changed his racket after Wimbledon, going for a bigger fram, was outplayed on the clay by the left-hander.
Delbonis, 22, had seven break points in the match, taking just the one in the opening set, as he punished Federer's second serve.
The 114th-ranked Argentinian, whose previous semi-final appearance was two years ago, also on clay and also in Germany, in Stuttgart advanced to his first career final after Federer made two straight groundstroke errors and netted a forehand on Delbonis' first match point.
"I feel like I'm dreaming," said Delbonis. "My key was to enjoy the match and enjoy playing this kind of player, because he's the best of all time."
Delbonis will next take on Fabio Fognini who remains on course for his second title in a fortnight after beating Nicolas Almagro 6-4 7-6 (1).
The 26-year-old won his first career title in Stuttgart last week and is riding a nine-match winning streak.
His clay-court form has been impressive as his latest victory was the 12th-seeded Italian's 24th win on the surface this season.
Third-seeded Almagro, who was runner-up in the tournament two years ago, came into the contest as favourite, but it was Fognini who prevailed following two tough sets of tennis.
Fognini is the second Italian to reach the final in Hamburg after Adriano Panatta lost to Manuel Orantes in 1972.