Seven-time winner Roger Federer lost in straight sets to 21-year-old Russian qualifier Andrey Rublev in the third round of the Cincinnati Masters.
The world number 70 won 6-3 6-4 in just over an hour to set up a quarter-final against compatriot Daniil Medvedev.
Defeat means he will have played just two matches in preparation, after an opening win over Juan Ignacio Londero.
On Rublev, Federer said: “He was excellent today. He didn’t give me anything. It was tough for me but an excellent match for him. I was impressed.
“I just would have hoped for an overall better performance. He was mixing it up nicely on the serve and on the return he was really consistent.”
In a first meeting between the pair, Federer initially broke back at 2-0 down in the first set but Rublev established a 4-1 lead from which the SWISs could not recover. He again had no answers as the Russian made the decisive break to lead 4-3 in the second.
After his biggest career win to date, Rublev said: “It is such an amazing feeling when you’re playing a legend like Roger and all these people supporting him until the end. One day I hope I’m going to feel the same.
“Today I was just trying to do my best and was thinking in my head that I need to play every point until the end no matter what.”