Roger Federer's return from a six-month injury layoff suffered a setback when German teenager Alexander Zverev shocked the Swiss great 7-6(1) 6-7(4) 7-6(4) in an entertaining Hopman Cup tie on Wednesday.
The 17-times grand-slam winner underwent knee surgery last February and, after missing the French Open with a back injury, announced in July that he needed extensive rehabilitation and would be sidelined for the rest of the season.
Representing his country in the mixed team event in Perth, Federer beat Briton Dan Evans on Monday but he faced stiff opposition from Zverev who enhanced his reputation by claiming his second win in three matches over the player many consider the greatest.
Federer served for the first set at 5-3 but the 19-year-old Zverev fought back and eventually rolled through the tie-breaker to claim the first set.
Federer, 35, took the second set in similar manner to force the decider but Zverev had the last laugh in the pulsating contest.
"To beat Roger, you really have to play an unbelievable match, from the first point to the last. I thought we both played really well," Zverev said in a courtside interview after putting Germany 1-0 up in the Group A tie.
"I always play unbelievably against him. It's always nerve-racking to play against him. I always play in front of a big crowd which helps me a lot as well," added Zverev, who also beat Federer in the Halle Open semi-finals last June.