World number one Novak Djokovic continued his march to a fifth China Open title by dismantling a flat Grigor Dimitrov in a one-sided quarter-final on Friday while women's top seed Serena Williams was forced to pull out through injury.

American world number one Williams, who had reached the last eight on the Beijing hard courts after a 6-1 1-6 6-2 win over Czech Lucie Safarova late on Thursday, cited a problem in her left knee for handing a walkover to Australian Sam Stosur.

It was the second tournament in a row the 33-year-old, who won her 18th grand slam singles title at the U.S. Open last month, had been forced to withdraw after she quit her opening match at the Wuhan Open last week because of illness.

Bulgarian Dimitrov looked off color in his display against the Beijing expert Djokovic, who extended his record on the blue hard courts to 22-0 with a 6-2 6-4 win.

The Serb targeted the faltering backhand of the fifth seed in the opening set, which he took with breaks in the fourth and eighth games in an improved display after showing rust in his opening matches.

The second set seemed to be going the same way as Djokovic pushed Dimitrov back with some deep groundstrokes, while producing his trademark extensive court coverage to negate the threat of his opponent as he broke to lead 3-1.

Dimitrov finally showed some fight, breaking back with a thumping forehand cross court winner only to lose serve again in the next game to trail 4-2.

Djokovic looked set to serve out the match at 5-3 and 30-0 up but Dimitrov rallied again, breaking the Serb with the point of the match after a stunning backhand winner down the line at full stretch.

But rather than capitalize on the heroics, the world number 10 crumbled, giving up the match on a third double fault to leave his bid to qualify for the ATP Tour World Finals in London next month in trouble.

Djokovic will face either U.S. Open champion Marin Cilic or Briton Andy Murray, who, like Dimitrov, is chasing a place in the season ending event for the top eight, in the last four.

In the women's tournament, Stosur will take on Wimbledon champion Petra Kvitova in the semis after the Czech beat Italian Roberta Vinci 7-6 (7) 6-4.

World number three Kvitova, who won last week's event in Wuhan, mixed seven aces with six double faults as she battled through.

French Open champion Maria Sharapova faces Russian compatriot Svetlana Kuznetsova in quarter-final action on Friday with former world number one Ana Ivanovic awaiting the winner in the last four.