The former world number one and reigning U.S. Open champion produced an uneven first set, with flashes of brilliance punctuating a string of unforced errors as he tried, and failed, to break down Canas's persistent baseline game.

Having smashed, volleyed and rifled an unreturnable serve past the Argentine in the fifth game to save three break points, Roddick then netted a routine forehand before relinquishing his serve on a double fault.

He broke back to level at 5-5 and force a tiebreak, but again proved wasteful -- throwing away three set points before scooping a half-volley into the net to hand the set to Canas.

A casual service game -- concluded on another double fault -- gave Canas a vital break in the opening game of the second set.

Though not in obvious physical difficulty, Roddick called for the physiotherapist to treat his left knee. It did no good and he suffered a second break as his first-serve percentage plummeted to 17 per cent and Canas held on to win.

"I think I played well today, especially on the return of serve," said Canas afterwards.

Asked whether he now felt he could challenge for the title, the world 80th-ranked player replied: "I'm trying to think about the later rounds at all. I just want to take each match as it comes".