Juan Carlos Ferrero's battle against injury this season showed no signs of easing as he was forced to retire in the first round of the Toronto Masters.
Ferrero endures the pain as he receives treatment on his back
The Spanish seventh seed was suffering from back pain against Fabrice Santoro and pulled out with his opponent leading 3-2 in the first set.
The Frenchman now faces Slovakian qualifier Karol Beck or Croatia's Ivan Ljubicic in the second round.
Ferrero's season has been wrecked by a rib injury and a bout of chickenpox.
"I hit a return and I started to feel it," said the Spaniard.
"When I was serving I felt I couldn't still play - I hope to be ready for Cincinnati, but I don't know," he said, referring to the Masters tournament before the US Open.
Andre Agassi defeated Tommy Haas 4-6 6-4 6-4, avenging a defeat in Los Angeles two weeks ago.
The German took the first set before Agassi rallied to win the match.
Haas double faulted in the ninth game of the third set to put his opponent 5-4 in front and Agassi held his serve to win the match.
"I felt good, I was running well," said Agassi "I'm trusting my legs out there, which is a good feeling again."