The former world No 1 lost 6-3, 2-6, 6-3 to eighth-seeded Petra Kvitova in the second round of the competition which lasted for two hours on Tuesday.
The mother of one was also knocked out in her last tournament at San Jose two weeks ago after reaching the Wimbledon final. Cincinnati was her fifth tournament since she had her baby last September.
Speaking after the game, the 36-year-old said: “You know, this is a long comeback. I just began. I just started, definitely at the very, very beginning. I’m getting there, and I’m going to just continue to work hard and, hopefully, I’ll start winning more matches.”
