LUXEMBOURG (AP) - Venus Williams, playing her first tournament since the U.S. Open, defeated ninth-seeded Mona Barthel 7-6 (4), 6-4 at the Luxembourg Open on Thursday despite six double-faults and four service breaks by her German opponent.

Williams struggled with a back problem at the U.S. Open.

Top-seeded Roberta Vinci of Italy reached the quarterfinals, beating Anne Keothavong of Britain 6-4, 6-1. Andrea Petkovic of Germany, ranked 182nd, defeated third-seeded and former No. 1 Jelena Jankovic of Serbia 7-5, 6-0.

Daniela Hantuchova of Slovakia downed Kirsten Flipkens of Belgium 6-4, 4-6, 6-2. Flipkens had won 24 of her previous 26 matches to give her a career-high No. 58 ranking.

In another second-round match, Ksenia Pervak of Kazakhstan beat sixth-seeded Sorana Cirstea of Romania 7-6 (4), 6-1.

SI Apps
We've Got Apps Too
Get expert analysis, unrivaled access, and the award-winning storytelling only SI can provide - from Peter King, Tom Verducci, Lee Jenkins, Seth Davis, and more - delivered straight to you, along with up-to-the-minute news and live scores.