Former champion Maria Sharapova made 62 unforced errors as she lost her first match since hiring Jimmy Connors as her coach.
American Sloane Stephens beat the world number three 2-6 7-6 (7-5) 6-3 at the Western & Southern Open.
Sharapova was making her return to tennis seven weeks after her last match, a shock early Wimbledon defeat.
“Every good round starts with a bogey – not the start we wanted, so back to work tomorrow.”
Sharapova split with coach Thomas Hogstedt after losing to world number 131 Michelle Larcher de Brito in the second round at Wimbledon.
A hip injury has kept the four-time Grand Slam champion off court since that defeat.
Sharapova enjoyed a first-round bye but struggled to find any rhythm as she lost to world number 15 Stephens.
She said: “I started the match off pretty well, but when you put yourself in a really good position you can’t let it go. That’s what I did tonight.
“I didn’t continue what I was doing well for the first set and a half, and that hurt me.
“I stopped being patient, I started making a lot more errors, especially off the first ball. Just errors that I shouldn’t make.”
BBC SPORTS
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