MEMPHIS, Tenn. — Jack Sock of the United States upset No. 2 seed Milos Raonic in opening-round play with a 6-3, 5-7, 7-5 victory Wednesday at the U.S. National Indoor Tennis Championships.

MEMPHIS, Tenn. — Jack Sock of the United States upset No. 2 seed Milos Raonic in opening-round play with a 6-3, 5-7, 7-5 victory Wednesday at the U.S. National Indoor Tennis Championships.

Raonic, who won the title at the SAP Open in San Jose last week, had finished as runner-up to Andy Roddick in 2011 and to Jurgen Melzer in 2012.

Sock won his first tour match of the year.

Sock broke Raonic to go ahead 6-5 in the final set and closed the match with an ace, his 11th of the match.

Other seeds advancing Wednesday were No. 6 Tommy Haas of Germany and No. 7 Alexandr Dolgopolov of Ukraine.

Haas, runner-up to Raonic last week, posted a 7-6 (6), 6-4 victory Wednesday over Russia's Evgeny Donskoy.

Haas, 34, the oldest player in the draw, is a three-time Memphis champion who won the first ATP Tour title of his career in Memphis 14 years ago.

In other first-round play, former world No. 1 Lleyton Hewitt of Australia survived two match points in the second set and defeated Yen-Hsun Lu of Taiwan 2-6, 7-6 (3), 6-4.

Hewitt, who turns 32 Sunday, is seeking his first Tour title since 2010.

In women's second-round matches, top seed Kirsten Flipkens of Belgium advanced with a 7-5, 3-6, 6-3 victory over Lesia Tsurenko of Ukraine and No. 3 Sabine Lisicki of Germany beat Melinda Czink of Hungary 7-6 (0), 4-6, 6-2.

No. 2 seed and defending champion Sofia Arvidsson of Sweden was eliminated by Marina Erakovic of New Zealand 6-1, 3-6, 6-3. Arvidsson defeated Erakovic in last year's final.

Dubai Championships

At Dubai, United Arab Emirates, defending champion Agnieszka Radwanska led several of the top seeds into the Dubai Championships quarterfinals on a day overshadowed by the withdrawal of Serena Williams because of a back injury.

Williams, who became the oldest top-ranked woman Monday, said she's had a sore back and ankle since the Australian Open, though she still reached the final in Qatar last weekend.

Her departure, announced 45 minutes before she was to face 11th-ranked Marion Bartoli, came a day after second-ranked Victoria Azarenka, who beat Williams in the Doha final, withdrew because of a right foot bone bruise.

Former No. 1 Caroline Wozniacki joined Radwanska in the quarters, brushing aside an error-prone Zheng Jie of China 6-1, 6-0. Former U.S. Open champion Sam Stosur beat Hseih Su-wei 6-4, 6-0.

Former Wimbledon champion Petra Kvitova also got past former No. 1 Ana Ivanovic 7-5, 7-6 (1).

The only upset was fourth-seeded Angelique Kerber, who racked up 10 double faults in falling to Roberta Vinci of Italy 5-7, 6-1.

Radwanska had a tougher-than-expected test in winning 7-5, 6-3 against Yulia Putintseva of Kazakhstan, who has only beaten two top 50 players.

Copa Claro

At Buenos Aires, Argentina, top-seeded David Ferrer of Spain began his defense of the Copa Claro title with a comfortable 6-4, 6-1 victory over Argentine wild card Agustin Velotti.

Ferrer, ranked No. 4, is a heavy favorite to win his second tournament of the season. He won in Auckland, New Zealand, last month.

The first-round match was postponed Tuesday because of rain in the Argentine capital.

Open 13

At Marseille, France, top-seeded Tomas Berdych struggled past Ernests Gulbis of Latvia 6-4, 6-7 (10), 6-4 to reach the quarterfinals of the Open 13.

Berdych withstood 19 aces from Gulbis in his second-round opener.

He was joined by third-seeded Jo-Wilfried Tsonga, who eased past Nikolay Davydenko 7-6 (1), 6-3.