Seventh seed Grigor Dimitrov lost in straight sets in the US Open second round to Russian 19-year-old Andrey Rublev.
The Bulgarian, 26, was a break up in both of the first two sets but eventually lost 7-5 7-6 (7-3) 6-3 at Flushing Meadows, New York.
Rublev, who beat British number three Aljaz Bedene in round one on Wednesday, will face Bosnia's Damir Dzumhur next.
Czech 15th seed Tomas Berdych lost in four sets to Alexandr Dolgopolov.
The Ukrainian world number 64 triumphed 3-6 6-1 7-6 (7-5) 6-2.
Berdych hit 57 unforced errors, 12 double faults and had a first-serve percentage of only 45%.