Reigning champion Marat Safin and French Open winner Rafael Nadal have pulled out of next week's Australian Open, the year's first grand slam.
Safin beat Lleyton Hewitt to win the title in 2005
The 25-year-old Russian is still recovering from a knee injury while Spaniard Nadal has a foot problem.
"I am very sad I cannot go to the Australian Open to defend my title as champion," said Safin.
Both players intend to return to action at the tournament in Marseille, France, on 13 February.
They join four-time champion Andre Agassi on the sidelines - the veteran American withdrew last week after failing to recover from an ankle injury.
Safin has not played since losing to Robby Ginepri in the quarter-finals of the Cincinnati Masters Series in August 2005.
Safin said: "I have been working very hard and my knee is a lot better but it would not be right to go to a Grand Slam and test my knee right now.
"I am sorry to all my Australian fans but I will be back next year."
Nadal, the 19-year-old world number two, has not played since October's Madrid Masters.
"The Australian Open is too important a tournament for me to go to without having an option to win it," he said.
He made the decision to withdraw from the year's first Grand Slam event after practising with Gustavo Kuerten for 90 minutes in Barcelona on Monday.
World number one Roger Federer, meanwhile, insists he has fully recovered from the serious ankle injury he picked up at the end of last year.
Federer lost to David Nalbandian at the Masters Cup final last November while struggling with his ankle, but won his first event of 2006 in Qatar last week.
"I'm very pleased about it because after Shanghai I was wondering how it was going to be," said Federer, who plays the Kooyong Classic this week.
"I'm happy to say I'll be playing at 100% again."
Federer is warming up for next week's Australian Open in Melbourne at the Kooyong exhibition, alongside Nalbandian, Andy Roddick, Gael Monfils, Ivan Ljubicic, Guillermo Coria, Tommy Haas and Nicolas Kiefer.