LONDON, June 27 (Xinhua) -- Serena Williams, seeking a third Wimbledon tennis title, and two-time runner-up Andy Roddick advanced to the third round of the grass court Grand Slam with straight-set wins on Wednesday.

Seventh seed Serena overcame a lackluster start to defeat big-hitting Australian Alicia Molik 7-6 (7-4), 6-3 while third-seeded Roddick hammered Thailand's Danai Udomchoke 6-3, 6-4,7-6 (7-3) after a short rain delay.

Former top 10 player Molik is one of the few players on the women's circuit who can trade power with Williams, but the 26-year-old is still on the comeback from a potentially career-wrecking ear infection and remains something of an unknown quantity.

Two-time Wimbledon champion Williams is a serious title contender but also of uncertain form as she came back from a breakdown in each set before clinching the hard-fought victory.

"I'm excited to get through today because I knew it wasn't going to be really easy at all," said Williams.

In the first set, Molik charged the net to secure a break for 3-1, but a resurgent Williams broke back in the seventh game and then held her won serve to level the score 4-4.

At 5-5 a loose game from the Australian saw Williams break for 6-5. Molik called on another big forehand to set up the tie-break before Williams held her nerve to serve out the set at the first opportunity with an ace.

Roddick's victory over Thailand's Danai Udomchoke on Centre Court was hammered out in an hour and 44 minutes, but at times the American was clearly frustrated by an inability to crush the opposition even more emphatically.

"It was kind of an up-and-down match, even the way I played," said the 24-year-old, "Mentally I was a little bit up and down."

The conditions were again wretched, with a cold wind swirling around the roofless stadium, making it an ordeal for players and spectators alike. Although Roddick ran up 15 aces, by his standards too many serves were off target or were routinely dealt with by an opponent ranked 114 in the world.

Danai's defeat completed a bad day for the Asian players. Japan' s Aiko Nakamura were thrashed 6-1, 6-2 by ninth seed Martina Hingis and China's Yan Zi, Hsieh Su-wei of Chinese Taipei and Ayumi Morita of Japan all bowed out in the first round of the women's draw.

Roddick was followed onto Centre Court by women's top seed Justine Henin. The Belgian, seeking a first Wimbledon title and seventh Grand Slam, eased into round three after a quick 6-0, 6-4 victory over Russia's Vera Dushevina, who's ranked 83rd and has already surpassed her best performance at Wimbledon by reaching the second round.

Also on Wednesday, sixth seed Jelena Ivanovic surged past Melinda Czink 6-0, 7-6 in a delayed first-round encounter.

But men's champion Roger Federer will have to complete unfinished business on Thursday after his second round match against Argentine Juan Martin del Potro was suspended following a torrential downpour.

The Swiss world number one was leading 6-2, 7-5, 2-0 when the players had to scurry back into the locker room in the late afternoon.