British number one Johanna Konta lost in three sets in the opening round of the Cincinnati Open to 20-year-old Belarusian Aryna Sabalenka.
Konta took the first set and was a break up in the second but was beaten 4-6 6-3 6-4.
The 27-year-old, who is ranked five places lower than Sabalenka at 39th in the world, committed 10 double faults on her way to defeat.
Konta’s US Open warm-up will continue at next week’s Connecticut Open.
The final Grand Slam of the year begins at Flushing Meadows on 27 August.
Serena Williams marked her return to action after missing the Rogers Cup in Montreal with a comprehensive 6-1 6-2 victory over Daria Gavrilova.
The former world number one and 23-time Grand Slam champion withdrew from the event in Canada citing “personal reasons” and later explained she was struggling with her emotions following the birth of her first child in September.
Williams suffered the worst loss of her career earlier in the month when she was defeated 6-1 6-0 by Konta, but said she knew she would play “a zillion times better”.
The American was much improved against Australian Gavrilova, hitting 27 winners and eight aces on her way to a comfortable win.
She will play Czech world number six Petra Kvitova in the next round.
Victoria Azarenka, another former world number one, beat Spain’s Carla Suarez Navarro in three sets while US Open champion Madison Keys beat fellow American Bethanie Mattek-Sands 3-6 7-6 (7-3) 6-4.