Serena celebrates her win against the Russian in Beijing
Serena Williams reclaimed the world number one ranking with victory against Ekaterina Makarova in the second round of the China Open in Beijing.
Williams, 28, beat the Russian 6-3 6-2 and takes over the top spot because she progressed further in the contest than current holder Dinara Safina.
Russian Safina, 23, lost to Chinese wildcard Zhang Shuai in round two.
Defending champion Jelena Jankovic was another first-round casualty, losing in three sets to China's Peng Shuai.
The Serbian, who retired from the final of the Pan Pacific Open in Tokyo with an arm injury, began confidently against Peng, taking the first set 6-4.
But Jankovic's heavily strapped arm could not take the strain as she served for the match, allowing Peng to break and take the second set 5-7 before the world number 52 closed out the third 2-6.
"It was very difficult and I couldn't really play my best tennis," said Jankovic.
"Despite that I had a chance to win in two sets but she took it to a third and my arm just didn't hold up. It's very disappointing."
American Williams needed only an hour to see off Makarova, 21, firing three aces and conceding one double-fault.
"Obviously I'm happy to be there because I've worked so hard this year to get there, I think I've earned it," said Williams.
World number 15 Maria Sharapova produced a sensational fightback from 5-2 down in the third set to beat ninth seed Victoria Azarenka 6-3 6-7 7-5.
Sharapova, who claimed her first title since returning from injury in Tokyo last week, looked set for a second-round exit, but won five successive games to clinch an enthralling final set in three hours.
"The match isn't over until we shake hands," said Sharapova. "I realised that I didn't want to be going home. I'm not leaving China that soon.
"So I just went forward and I kept being aggressive and stayed positive and that gets you a long way."
Fourth seed Elena Dementieva, who won Olympic gold at the National Tennis Center last year, secured a 6-4 6-3 victory over Hungary's Melinda Czink.
The Russian faces home favourite Li Na in the last 16.