World No. 6 Andy Roddick suffered a knee injury that forced him to withdraw from his clash with Swiss No. 2 Stanislas Wawrinka in the second round of the ATP event in Shanghai.

Roddick was leading 4-3 in the first set when he fell and hurt his knee. As a result he was left with no option but to retire from the match.

It is the second time that Roddick has had to retire against Wawrinka; last year in the semifinals of the ATP Masters 1000 Event in Rome, the American also had to retire when trailing 3-0 in the first set.

Wawrinka shocked the tennis world earlier this year when he beat his compatriot, the world No. 1 Roger Federer, 6-4, 7-5 in Monte Carlo.

Roddick still has a decent chance of qualifying for the year-end Masters Cup in London next month, and he will be hoping that this knee injury does not keep him out of action for too long. It would take a brave person to Bet against Roddick winning the Masters Cup for the first time.