A doctor's medical report showed the French Open champion and top seed had developed a cyst between bones in his right foot. Earlier it was reported that he had injured his Achilles tendon.
Muster sought treatment after beating Germany's Martin Sinner 6-2, 6- 3 in his first match of the competition.
The Austrian, who won the Stuttgart Open on Sunday, is expected to be treated with anti-inflammatory drugs and may have to undergo an operation to remove the cyst.
Muster became the first Austrian to win a Grand Slam title, confirming his status as the world's best clay-court player by beating the American Michael Chang in the final of the French Open last month.
Jim Courier was made to fight hard before overcoming a tired Daniel Nestor, of Canada, 7-6, 6-4 in the first round of the Canadian Open in Montreal. Courier, ranked 13th, struggled to overcome his 150th-rated opponent - even though the Canadian, who has played non-stop for the past seven weeks, was not in peak condition.
It did not appear to be that hot, but Nestor said he felt that the conditions were oppressive and that he started to cramp in the eighth game of the second set. "I felt the heat a lot and almost had to stop. I didn't want to stop because it wouldn't have looked too good - I started cramping at 4-4 but I just kept going for it," he said.
Courier said: "I hope to get in as many matches as I can this week. I just expect to compete hard all summer and see how my results go."
The 14th seed, David Wheaton, was defeated by Brett Steven of New Zealand 6-4, 3-6, 6-3. Steven, who made 70 per cent of his first serves, felt this helped him through the crucial third set.
The Dutchman Richard Krajicek, ranked 14th in the world, successfully came through the qualifying event after failing to secure one of the wild cards which ultimately went to the seventh-ranked Goran Ivanisevic and four Canadian players.
Krajicek had not planned on playing in the event. But after losing in the first round at Wimbledon he was eager for the competition and did not resent having to qualify.
Tim Henman overcame Australia's Grant Doyle to move into the last 16 of the Northern Electric Open at Newcastle yesterday. The British No 5, from Oxford, went a set behind but fought back to clinch a 3-6, 6-1, 6- 4 win over the former junior world No 1.
Henman, who suffered a surprise second-round defeat in last week's Manchester Open, also suffered a scare in the third set before sealing victory in 1hr 53min. "I knew it was going to be tough. We came through the junior ranks together and at the time he was the best in the world," Henman said.
The Britons Andrew Richardson and Miles Maclagan both suffered first- round defeats.Reuse content