SHANGHAI — Gael Monfils of France and Stanislas Wawrinka of Switzerland joined the long list of injury withdrawals from the Shanghai Masters, pulling out of their matches Thursday.

Monfils, seeded 11th, withdrew with a back injury while trailing unseeded Croatian Ivan Ljubicic 6-2, 3-0.

Wawrinka was treated on court for stomach pain and trailed 3-6, 7-6 (5), 4-2 against Czech Radek Stepanek when he retired.

Five other players have retired from matches with assorted injuries at Shanghai this week: reigning U.S. Open champion Juan Martin del Potro, Andy Roddick, Tommy Haas, Jose Acasuso and Mischa Zverev.

Sixth-seeded Nikolay Davydenko of Russia, who himself withdrew from two tournaments earlier in the year with a heel injury, advanced to the quarterfinals with a 6-3, 7-5 win over 10th-seeded Fernando Gonzalez on Thursday.

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

Davydenko will play Stepanek in the quarterfinals, while Ljubicic will face either top-seeded Rafael Nadal or Tommy Robredo in the last eight.