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, 6-3, 6-4 to 16th-seed Lucie Safarova in the first round of the Western and Southern Open.
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-time 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.
"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.
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.
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.