SHANGHAI — Rafael Nadal and Novak Djokovic had straight-set wins Thursday to advance to quarterfinals of the Shanghai Masters on a day when two more players retired with injuries.

The top-seeded Nadal, aiming for his first title since winning the Rome Masters in May, defeated Tommy Robredo 6-1, 6-4.

"I started the second set bad," said Nadal, who won five tournaments this year, including the Australian Open. "So for that reason, later I had to play all the set under pressure."

Djokovic moved into the quarterfinals by stopping German qualifier Rainer Schuettler 6-4, 6-2, closing out the match with back-to-back aces.

"It was comfortable," Djokovic said. "I didn't play the last couple of games of the first set that well, but the important thing is that I played as much as I needed to win."

In the quarterfinals, Nadal will face Ljubicic and Djokovic will play Gilles Simon.

Gael Monfils of France and Stanislas Wawrinka of Switzerland joined the long list of players to retire in matches this week. Monfils stopped while trailing Ivan Ljubicic of Croatia 6-2, 3-0 because of a bad back. The unseeded Wawrinka was treated on court for stomach pain and retired when trailing Radek Stepanek of the Czech Republic 3-6, 7-6 (5), 4-2.

Five other players have retired from matches during the tournament — U.S. Open champion Juan Martin del Potro, Andy Roddick, Tommy Haas, Jose Acasuso and Mischa Zverev.

Sixth-seeded Nikolay Davydenko of Russia also advanced to the quarterfinals, defeating 10th-seeded Fernando Gonzalez 6-3, 7-5.

Davydenko, who pressured the Chilean with his return game, has won 12 of the 13 sets he has played against Gonzalez in their six career meetings.

Ninth-seeded Robin Soderling of Sweden advanced by beating fifth-seeded Jo-Wilfred Tsonga of France 6-3, 6-3.

Davydenko will meet Stepanek in the quarterfinals and Soderling will take on Feliciano Lopez.