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 when trailing the unseeded Croatian Ivan Ljubicic 6-2, 3-0.

Wawrinka was treated on court for stomach pain before retiring when trailing Czech Radek Stepanek 3-6, 7-6 (5), 4-2.

Seven 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, Mischa Zverev, plus Monfils and Wawrinka.

Sixth-seeded Nikolay Davydenko of Russia, who himself withdrew from two tournaments earlier in the year with a heel injury, progressed to the last eight with a 6-3, 7-5 win over 10{+t}{+h}-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.

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