Roddick receives on-court treatment for his injury before retiring
Fourth seed Andy Roddick limped out of the Shanghai Masters after retiring in his second-round match against Stanislas Wawrinka on Tuesday.
The 27-year-old American, who only a day before had questioned the length of the ATP season, injured his knee while leading 4-3 in the first set.
"It was enough to make me stop a match - beyond that I don't know, that's not my profession," said Roddick.
Roddick has not yet qualified for the World Tour finals in London next month.
On Monday, Roddick called for the ATP to allow top players a break in the season or risk curtailing the careers of players like Roger Federer and Andy Murray, who are absent from Shanghai because of fatigue and a wrist injury respectively.
"Trust me, as frustrated as the Chinese fans are that we're not here, I promise you we're more frustrated," added Roddick.
Sixth seed Nikolay Davydenko was the first man to reach the third round, seeing off Igor Kunitsyn 6-4 6-2.
Australian Leyton Hewitt also eased past big-serving American John Isner 6-2 6-4.
Hewitt is almost a foot smaller than 6ft 9in Isner, but he restricted the right-hander to only five aces.
"It's a matter of just taking your chances when you get into it against those guys, and I did that exceptionally well today," said the 28-year-old.
Hewitt will play Gael Monfils after the 11th seed took the honours in one of several matches between compatriots, crushing fellow Frenchman Paul-Henri Mathieu 6-2 6-2 in 54 minutes.
Chile's Fernando Gonzalez, currently in the eighth and final spot in the race to the World Tour Finals, will play Davydenko in the last 16 after beating Brazilian qualifier Thomaz Bellucci 6-3 6-4.
Swede Robin Soderling is another in the hunt for a place in London and the ninth seed helped his cause with a 6-3 6-4 first-round win over Romanian Victor Hanescu, while Tommy Haas beat Benjamin Becker 7-6 (5) 6-4 in an all-German match.