Former world number one Serena Williams continued her return to form with a convincing 6-3 7-6 victory over Czech Lucie Hradecka in the first round of the Cincinnati Open yesterday but showed signs of fatigue in her second set.

Williams, who clinched the Toronto Cup on Sunday, is looking for her third tournament win since coming back from an 11-month injury lay-off in June.

The American will next face the woman she beat in Sunday's final -- Australian Samantha Stosur -- in the second round.

Tenth-seeded Stosur brushed aside Eleni Daniilidou of Greece 6-3 6-1 in her opening match.

Williams looked in good form in the first set as she pounded her serve against the 49th-ranked Hradecka, who came into this week on a six-match losing streak.

However the American, who was 5-1 up in the second, struggled to finish off her opponent and showed some signs of tiredness before sealing victory in the tie-break.

"I was definitely up, I didn't close it out and she started playing differently and better and running a lot," Williams said after recording her 12th consecutive win. "I was just struggling at that point.

"I haven't played in almost a year, and even the year before I didn't play that much. But I would rather have that than fresh legs from not winning."

With the US Open just a fortnight away, Williams said she was wary of pushing herself too hard before the last grand slam event of the season.

"I'm going to have to figure that out," she added. "I definitely don't want to overdo it. My main goal right now is obviously to do well.

"Nothing against Cincinnati or Toronto or the Stanfords that I played, but this is all preparation. Everyone is preparing for the US Open, so I have to be smart and make sure it's not too much," Williams said.

World number five Li Na of China won her second round match against Czech Lucie Safarova 6-3 6-4 while eighth seed Marion Bartoli of France beat Ukrainian Alona Bondarenko 6-1 6-2.

In the first round, Italian Flavia Pennetta enjoyed a surprise 2-6 6-3 6-3 win over Russia's 12th-seeded Anastasia Pavlyuchenkova.

Another Russian, world number seven Maria Sharapova, was set to face Australian Anastasia Rodionova later yesterday.