Venus Williams failed to sustain the momentum from her run to last week's Montreal final and ran out of gas in Cincinnati as she fell 6-7(2) 6-3 6-4 to 16th seed Lucie Safarova in the first round of the Western and Southern Open on Tuesday.
Williams, who beat her top-ranked younger sister Serena to reach the Rogers Cup final, where she lost to Poland's Agnieszka Radwanska, looked like she might be ready to make another run in Cincinnati after taking the opening set in a tiebreak.
However, the seven-times grand slam champion could not convert any of her four break chances in the second while Safarova converted her one and only opportunity to level the match.
Czech Safarova, a Wimbledon semi-finalist, stepped up the pressure in the third with two more breaks to clinch her first win in five career meetings against the 34-year-old American.
"It was definitely a quick turnaround, maybe would have been a little better to play a little later in the day," said Williams, back in top 20 for first time since March 2013 following her runner-up finish at Montreal.
"But she just played so well. No matter what shot I hit she hit a winner, or if I hit it really deep she somehow managed to control it down the line."
Fifth seed Maria Sharapova needed three sets to dispose of American Madison Keys in her evening second round contest, finally prevailing 6-1 3-6 6-3 in just under two hours.
In the other feature second round contests Denmark's Caroline Wozniacki made short work of China's Zhang Shuai 6-2 6-3 win and second seed Simona Halep won 6-4 6-2 over Kirsten Flipkins.
Wozniacki, a title winner last month in Turkey, was a class above the world number 31 Zhang, who was bounced out in the first round of her previous four tournaments.
Halep, playing her first match since moving to a career high world number two, saved seven of nine break points on the way to victory in 87 minutes.
Earlier Spanish 15th seed Carla Suarez Navarro, an upset winner over Sharapova in Montreal, was also made to work for her spot in the second round, needing three-sets to tame Bulgarian Tsvetana Pironkova 6-3 1-6 6-2.
Also in the first round, American Sloane Stephens beat German Andrea Petkovic 6-3 6-3 and 14th seed Italian Sara Errani beat qualifier Yanina Wickmayer 7-5 3-6 7-6(2). (Reporting by Steve Keating and Ben Everill; Editing by Peter Rutherford)