Roger Federer suffered a shock loss to Russian qualifier Andrey Rublev in the third round of the Western and Southern Open in Cincinnati.
Rublev, the world No 70, broke Federer three times as he powered to a stunning 6-3 6-4 win in 62 minutes.
The defeat was a blow to 20-time major champion Federer's US Open preparations.
Rublev will now play compatriot Daniil Medvedev in the quarter-finals after the biggest win of his career.
World No 3 Federer said on www.atp.com: "He was super clean - defence, offence, serving well. Didn't give me anything. He was everywhere.
"So it was tough for me, but an excellent match by him. I was impressed."
After his victory, 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."
Meanwhile, Novak Djokovic beat Pablo Carreno Busta in straight sets, 6-3 6-4.
Djokovic will next play Lucas Pouille in the quarter-final.