Defending champions Andy Roddick and Jelena Jankovic were both upset at the 6.6-million-dollar tournament, a combined ATP/WTA event aimed at bolstering China's position as a key tennis marketplace.

Russians Maria Sharapova and Elena Dementieva advanced to the round of 16 in the women's draw, as did their countryman Marat Safin, on a day of blinding A-list star power at the Olympic tennis centre.

Williams, the second seed here and current world number two, breezed past Russia's Ekaterina Makarova, 6-3, 6-2, into the round of 16, ensuring that her name will be atop the WTA rankings when the new list is issued on Monday.

The path to the top spot was cleared for the 28-year-old Williams, who last held the top ranking in April, when current number one Dinara Safina crashed out of the tournament on Monday, losing to Chinese wild card Zhang Shuai.

"It feels pretty good. I'm really excited. I don't want to put too much pressure on myself. I'm obviously happy to be there because I feel like I've been working so hard all year," said Williams.

Nadal, the top seed and world number two, held off a fierce challenge from former Australian Open finalist Marcos Baghdatis, winning in three sets, 6-4, 3-6, 6-4.

The hard-hitting baseliners remained even throughout much of the match, delighting the crowd with long rallies and hitting amazing angles with their ground strokes.

The 23-year-old Nadal got the upper hand by taking the first set, but the Cypriot, who won the China Open in 2006, bounced right back to take the second.

At 4-4 in the third set, the left-handed Spaniard pounced, breaking Baghdatis and then serving out the match, exulting when the Cypriot's final stroke landed in the net.

"I'm very happy with how I finished -- I was playing aggressive and going to the net a lot of times, so that's important," said Nadal.

"The only way to be at the top is to play aggressive."

He praised Baghdatis, who has fallen to number 90 in the world rankings from a career-high number eight, saying he was always a "very dangerous player".

"Marcos served really well at important moments," he said. "I am happy to see him back here."

Roddick, the third seed here, stumbled out in straight sets to Polish qualifier Lukasz Kubot, 6-2, 6-4, while China's Peng Shuai defeated the eighth seed Jankovic, 4-6, 7-5, 6-2.

"I didn't play well," said Roddick.

"I don't know if I was super-prepared and it showed. I was trying stuff out there and nothing seemed to be working too well."

Jankovic said an arm injury that forced her to retire from the Pan Pacific Open final in Tokyo against Sharapova at the weekend was still troubling her.

"It was very difficult and I couldn't play my best tennis," she said.

Sharapova, a three-time Grand Slam winner, fired 13 aces in her gutsy three-set come-from-behind win over ninth seed Victoria Azarenka of Belarus, 6-3, 6-7 (5/7), 7-5.

"I certainly felt like I stepped it up when I needed to," Sharapova said.

In other women's matches, fourth seed Dementieva, fellow Russian seventh seed Vera Zvonareva, 10th seed Flavia Pennetta of Italy and 14th seed Marion Bartoli of France, all moved into the round of 16.

In the men's draw, fourth seed Nikolay Davydenko, fifth seed Fernando Verdasco, seventh seed Fernando Gonzalez and Safin, the 2004 China Open champion, all won their opening matches.