Based on pre-tournament calculations, Williams moves to the top ranking by finishing better than current No. 1 Dinara Safina at the tournament. Safina lost to local wild card entrant Zhang Shuai in the second round on Monday.
Williams needed just over one hour for the relaxed victory in which she fired three aces and conceded just one double fault.
The WTA reported that Safina's upset loss made No. 226 Zhang the lowest ranked woman ever to defeat a world No. 1. Frenchwoman Julie Coin held the previous record, having a No. 188 ranking when she beat then No. 1 Ana Ivanovic at last year's U.S. Open.
Former world No. 1 Maria Sharapova rallied to keep her impressive comeback alive with a thrilling 6-3, 6-7 (5), 7-5 win over Victoria Azarenka of Belarus.
Sharapova, who last week won the Toray Pan Pacific in Tokyo for her first tournament victory since returning from a 10-month injury layoff, recovered from a 2-5 deficit in the final set, winning five games in a row as ninth-ranked Azarenka struggled to find a response.
After winning the second set tiebreak, a confident Azarenka broke Sharapova three times to take a commanding lead. The tide turned in game eight, when Sharapova won a break of her own and then made a stand that proved unassailable.
Fourth-seeded Russian Elena Dementieva enjoyed an easy 6-4, 6-3 win over Melinda Czink of the Czech Republic.
In men's play, Safina's older brother Marat Safin enjoyed enthusiastic support from the Chinese fans on his way to a comfortable 6-4, 6-2 win over the unseeded Jose Acasuso of Argentina.
Winner of the inaugural China Open in 2004, two-time Grand Slam winner Safin is making his final appearance at the event after announcing plans to retire at the end of the year.
American James Blake downed Germany's Florian Mayer 6-3, 3-6, 6-3 in another first-round match, while No. 2 Rafael Nadal and defending China Open champion Andy Roddick were to open their campaigns later on Tuesday.