Naomi Osaka suffered a first-round exit at the Western & Southern Open with a
6-4 7-5 defeat to Zhang Shuai.
Osaka, the 2020 finalist in Cincinnati, lost in 75 minutes as Zhang levelled the pair's head-to-head record at three wins apiece.
Zhang had not defeated a major winner since the first round of the 2020 Australian Open when she beat Sloane Stephens in three sets.
Former world No 1 Osaka served well by landing 10 aces and only lost 10 points behind her first delivery.
But Zhang, who is ranked 44, capitalised on Osaka's 29 unforced errors to turn the match her way.
Osaka showed flashes of her best form to level the second set at 5-5, having trailed 5-3, producing some excellent backhands to break Zhang for the only time.
But Osaka's backhand let her down at 5-5 and Zhang made no mistake when serving for the match a second time with a love hold sealed by her fourth ace.
Karolina Pliskova, the 14th seed and 2016 Cincinnati champion, eased into the second round with a 7-5 6-1 victory over Venus Williams.
The two former world No 1s were facing off for the first time in five years and Pliskova scored her third win in four attempts against Williams.
Romania's Sorana Cirstea knocked out 12th seed Belinda Bencic 6-2 6-7 (3-7) 6-4 in a contest which lasted two hours and 38 minutes.
Belgium's Elise Mertens brushed aside Ukraine's Anhelina Kalinina 6-1 6-1 to set up a second-round date with Pliskova.