Andy Roddick's body finally gave up on him when he limped out of the Shanghai Masters in the first set of his second-round match against Swiss Stanislas Wawrinka on Tuesday.
The fourth seed, who launched a scathing attack on the gruelling tour schedule on Monday, was 4-3 up and looking to convert a break point when he pulled up short chasing a Wawrinka backhand and had to retire.
"It was enough to make me stop a tennis match ... beyond that I don't know, that's not my profession," the 27-year-old Roddick said of his left knee injury.
The American, who retired with an injured shoulder in his only previous tour clash with the Swiss, also lost the first match of his title defence in Beijing last week.
On Monday, he called for the ATP to allow top players a break in the season or risk curtailing the careers of "stars" 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.
The world number six still has to secure his place at next month's season-ending championship in London.
"I think regardless of what I do from here on, I still have a pretty good shot," said Roddick. "I don't really know the numbers but I can't really worry about that because it's out of my hands right now."
Former world number one Lleyton Hewitt cut big-serving John Isner down to size with a 6-2 6-4 victory in the first round of the $3.24 million event.
Back at number 23 in the world after sliding outside the top 100 following hip surgery, Hewitt is nearly a foot shorter than the 6-foot-9 (206-cm) Isner but came up with some fine returns to disarm the American.
Hewitt next plays Gael Monfils after the 11th seed took the honours in one of several matches between compatriots on Tuesday, crushing fellow Frenchman Paul-Henri Mathieu 6-2 6-2 in 54 minutes.
Sixth seed Nikolay Davydenko was the first man to reach the third round when he comfortably beat fellow Russian Igor Kunitsyn 6-4 6-2.
Davydenko reached the final of the last version of the Masters Cup tour championship here last year and is in the running for one of the three spots still up for grabs at the new season finale in London.
Chile's Fernando Gonzalez, 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.