Serena Williams appears to be rounding into form for the U.S. Open.

The 13-time Grand Slam champion posted a 6-4, 6-2 victory over Australian Samantha Stosur to win the Rogers Cup on Sunday.

Competing in just her fourth tournament since being sidelined for a year due to injuries, Williams broke her opponent's serve three times while facing only one break point. She concluded the 78-minute match by hammering her ninth ace.

Williams, who won two weeks ago in Stanford, secured her 39th career singles title -- third-most among active players -- and improved to 11-0 on hard court this year. The unseeded American also won the Rogers Cup in 2001.

The 10th-seeded Stosur fell to 2-9 in WTA finals. She had won two of the previous three encounters with Williams, including a victory in the quarterfinals of the 2010 French Open.