Novak Djokovic of Serbia hits a return against Zhang Ze of China during their men's singles match at the China Open tennis tournament in Beijing, China, October 8, 2015. [Photo/Agencies]

In other men's matches, Fabio Fognini reached the last eight after a 6-2 2-6 6-2 win against number seven seed David Goffin while Pablo Cuevas of Uruguay beat Ivo Karlovic 7-6(5) 7-6(7).

In the women's draw, Spanish rising star Garbine Muguruza made her 22nd birthday one to remember by sealing her spot for the WTA Finals in Singapore with a nervous 1-6 6-2 6-1 win over Mirjana Lucic-Baroni.

"I was really nervous before the match, because I knew I could get to Singapore and it was also my birthday," the 2015 Wimbledon finalist said.

But US Open champion Flavia Pennetta had her hopes of qualifying for the season-ending tournament in Singapore dented when she lost 3-6 6-4 6-3 to Anastasia Pavlyuchenkova.

Pennetta, who is engaged to Fognini, has already announced her decision to retire at the end of the season.

Her compatriot and opponent in the US Open final, Roberta Vinci, also went out with a 6-1 3-6 6-2 defeat to American qualifier Bethanie Mattek-Sands.

Pavlyuchenkova's opponent in the quarter-finals will be Ana Ivanovic who edged out Svetlana Kuznetsova 7-5 4-6 6-2 in a thrilling battle.