NEW YORK - Three-time champion Serena Williams and No. 1 seed Caroline Wozniacki were lopsided second-round winners Thursday at the U.S. Open.

Williams, the former world No. 1 who is seeded 28th, flattened Michaella Krajicek of the Netherlands 6-0, 6-1 in a mere 49 minutes at Arthur Ashe Stadium, closing out the uneventful affair with her 10th ace on a breezy afternoon in Flushing.

Wozniacki, meanwhile, needed only 63 minutes to knock off Dutchwoman Arantxa Rus, who won two breaks in the first set but never held serve in a 6-2, 6-0 defeat.

A runner-up two years ago, Wozniacki is still seeking her first Grand Slam title. The 21-year-old Dane moved on to face American Vania King, an easy 6-2, 6-0 winner Thursday over Australia's Jarmila Gajdosova.

Williams, 29, is 14-0 since Wimbledon -- all on hardcourts, including a pair of titles. She was playing a day after her sister, Venus, pulled out of the championship and disclosed she was suffering from an autoimmune disorder.

"Honestly, I just hope to keep my errors down and keep making my shots, and hopefully I will be able to win five more matches," said Serena, a 13-time Grand Slam champion. "If not, I'm still happy to be here."

Williams advanced to face fourth-seeded Victoria Azarenka of Belarus, who beat Argentina's Gisela Dulko 6-4, 6-3.

By beating Krajicek, who is the half-sister of former men's Wimbledon champion Richard Krajicek, Williams moved to 20-1 lifetime against non top-50 players.

Seventh-seeded gritty Italian Francesca Schiavone rolled into the third round with a 53-minute 6-1, 6-1 pasting of Croat Mirjana Lucic, while 10th-seeded rising German Andrea Petkovic overcame diminutive Chinese Zheng Jie 3-6, 6-3, 6-3. The 31-year-old Schiavone was this year's French Open runner-up and last year's French Open champ.

Up next for Schiavone will be South African Chanelle Scheepers.

Former U.S. Open runner-up Jelena Jankovic advanced on Day 4 of the fortnight with a 6-3, 6-4 handling of former top-10 player Jelena Dokic of Australia. The former world No. 1 Jankovic, of Serbia, was the 2008 U.S. Open runner-up to Williams.

Jankovic has been nursing a sore back.

"I have a day off tomorrow, and, you know, I will just try to receive some treatment and feel the, you know, the best as possible for my next match," Jankovic said. "I think I will be okay."

Former U.S. Open champ Svetlana Kuznetsova was an easy 6-4, 6-3 winner against Britain's Elena Baltacha 6-4, 6-3. The 15th-seeded former top-five Russian star Kuznetsova is a two-time Grand Slam champion, including a U.S. Open championship back in 2004. She also reached the final in 2007.

Another former No. 1 from Serbia, Ana Ivanovic, moved on without even lifting her racquet on Thursday.

The 16th seed received a walkover into the third round when last week's New Haven runner-up, Petra Cetkovska of the Czech Republic, pulled out of their scheduled encounter due to a quadriceps injury. Ivanovic, who is a former French Open champ and Aussie Open runner-up, will get Sloane Stephens in the second round after the American beat 23rd-seeded Shahar Peer of Israel 6-1, 7-6 (7-4).

Meanwhile, 17th-seeded Russian Anastasia Pavlyuchenkova held off Croat Petra Martic 6-4, 2-6, 6-4, 18th-seeded Italian Roberta Vinci vaulted past France's Alize Cornet 6-2, 6-3, and little-known Spaniard Silvia Soler-Espinosa dismissed 31st seed Kaia Kanepi of Estonia 6-4, 6-2 on the grounds at the USTA Billie Jean King National Tennis Center. Pavlyuchenkova should have her hands full with Jankovic in her next outing.

In battles featuring non-seeds, Scheepers handled German Mona Barthel 6-4, 7-5, Spain's Carla Suarez Navarro bested Romanian Simona Halep 3-6, 6-2, 6-2, and Uzbekistan's Akgul Amanmuradova defeated France's Pauline Parmentier 6-3, 7-5. The 19-year-old Halep stunned French Open champion and Aussie Open runner-up Li Na in her opening match this week.