WASHINGTON - Top seed Serena Williams beat her 40-year-old elder sister Venus Williams 3-6, 6-3, 6-4 on Thursday in a round-of-16 match at the WTA Top Seed Open in Lexington, Kentucky.

It was their 31st official meeting, in which Serena recovered from a set down to reach the quarterfinals in two hours and 19 minutes.

"I've lost a few tight sets lately, so I was telling myself I really wanted to win this one and try to focus on those last two games," she said in her on-court interview.

The 23-time Grand Slam winner also said the victory will boost her confidence heading into the US Open.

Serena, turning 39 next month, will next face Shelby Rogers, a 6-2, 7-5 winner over Canadian qualifier Leylah Fernandez.

Jil Teichmann advanced to the quarterfinals with a 6-2, 6-2 victory over No. 5 seed Yulia Putintseva on Thursday.

In another round-of-16 match, Catherine Bellis ousted Jessica Pegula 6-3, 6-2 in one hour and 25 minutes.