Andre Agassi has warned the tennis authorities the calendar must be overhauled or players risk burn-out.
Agassi and four other top players, including world number two Rafael Nadal, have pulled out of the Masters Cup in China due to injury.
"It's been terrible this year for the number of players who haven't been ready to play here," he said.
"It's a sign things must change for guys to feel better about their ability to get ready for an event this big."
 |  The ball's faster, guys are stronger, the movement is much more violent now - this means more injuries |
Nadal, who was unable to start his game against Gaston Gaudio because of a sore foot, echoed Agassi's concerns, and said in future he may seek medical guidance over how many tournaments to play.
"If the doctor says it's safe, I will play the same next year," he said.
"If he says play less, I will play a little bit less. The calendar is very, very tough."
Agassi, 35, was competing in the end -of-season event for a record 13th time, and he said the physical demands of top level tennis had changed a lot in his time in the game.
"The ball's faster, guys are stronger, the movement is much more violent. This means the potential for more injuries," he said.
"As the athleticism of the game improves, maybe there has to be some adjustments that make it possible for guys to go 100% all season.
"Somebody or something has to truly represent the better interest of the sport."
MMIX
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