Sixth seed Juan Martin del Potro became the first player to reach the quarter-finals of the Shanghai Open on Thursday after Tommy Haas withdrew injured.
German Haas, who beat compatriot Daniel Brands and American Sam Querrey to reach the third round, had been unable to play due to a back injury, according to media reports.
Argentine Del Potro has also had health problems during the tournament, battling fever during his second round match against German Philipp Kohlschreiber, which he rallied to win 3-6 6-3 7-6(4).
Frenchman Jo-Wilfried Tsonga also advanced to the next round by beating Japan's Kei Nishikori 7-6(5) 6-0.
Serb Novak Djokovic, who was replaced by Rafa Nadal as world number one on Monday, plays Italian Fabio Fognini while Roger Federer meets Frenchman Gael Monfils in their third-round encounters also on Thursday.
Spaniard Nadal meets Argentine Carlos Berlocq in the day's late match.