World number two Novak Djokovic raced into the last four of the China Open in straight sets on Friday as home favourite Li Na set up a testing women's semi-final with Russian tennis queen Maria Sharapova.

The 25-year-old Serbian thumped Jurgen Melzer of Austria 6-1, 6-2 to maintain his unbeaten record at the Beijing tournament and continue his push to finish the year at number one in the rankings.

Djokovic, behind only Roger Federer in the ATP standings, has won the China Open on the two occasions he has entered - in 2009 and 2010 - but missed last year because of injury.

The number one seed made light work of Melzer, winning in just 53 minutes, to set up a semi-final clash with Germany's Florian Mayer, who beat wildcard Zhang Ze to bring the Chinese wildcard's fairytale run crashing to a halt.

The 22-year-old, ranked 165th in the world, made history in the last 16 when he became the first Chinese man ever to beat a top 20 player, dumping out France's Richard Gasquet.

His defeat of the Frenchman, ranked 14th, also meant Zhang was the first men's player from China to reach the quarter-final of an ATP event since Pan Bing made the semi-finals in Seoul in 1995.

But matching Pan's 17-year-old mark proved a step too far for the right-hander from Nanjing, with Mayer winning 6-3, 6-4.

Third seed Jo-Wilfried Tsonga of France defeated Russia's Mikhail Youzhny 6-3, 6-2 to reach the semi-finals, where he will face Spain's Feliciano Lopez, who beat American Sam Querrey 4-6, 6-4, 6-4.

In the women's draw Li, Asia's only Grand Slam singles winner, beat defending champion Agnieszka Radwanska of Poland 6-4, 6-2 to join Sharapova and Victoria Azarenka in the semi-finals.

Victory for world number eight Li also secured her the final place at the end-of-season WTA Championships in Istanbul later this month.

"I have already qualified to play in the WTA finals so what I'm going to do is enjoy the match with Sharapova," said Li.

Showing ominous form, French Open champion Sharapova, yet to drop a set in China, won the first set 6-0, and was 3-0 up in the second when Germany's Angelique Kerber quit the match citing an injury to her right foot.

The Russian stayed on track for a re-match of this year's Australian Open final, which she lost to Azarenka, who has also yet to drop a set this week. Azarenka also came out on top in their semi-final clash at the US Open.

"I thought we played a high level of tennis in the first set and a half, and even though the scoreline was 6-0, 3-0 there were many games that went to deuce," Sharapova said.

"She had a few break points and long games on her serve," the Russian added, saying Kerber was probably lacking fitness at the end of a long season.

"I actually didn't see much of her injury. I was surprised she called the trainer and even more so when she retired quickly after that," Sharapova said.

"The schedule takes its toll on everyone. It's a normal process. If you feel like it's too bad, you've got to take care of it."

Azarenka, from Belarus, maintained her scintillating form, dismissing Romina Oprandi 6-2, 6-0.

"It's never easy to go in the tennis court and win the match even though the score was like this," Azarenka, 23, said.

"It's important to stay tough and have that concentration to make sure that you can win every ball and every point. I felt like I was playing well, but it wasn't easy," she added.