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“It’s just _ our sport has evolved into such a strong sport where physically _ I mean, it’s so much more demanding on the body and how we play.”

Clijsters said players were spending more time in the gym to build strength to cope with the increasing demands on their bodies and many traveled with a physical therapist.

“When I came on tour … nobody was hardly ever in the gym besides warming up with a skipping rope or doing some shoulder exercises and now there’s everybody _ because it’s necessary,” she said. “The tennis that I play is physically so demanding on the body and then … combine that on a hard court.”

Murray, who will play in a men’s semifinal at Brisbane on Saturday, dropped the opening sets in his first two rounds as he overcame soreness and stiffness from the offseason. He breezed through a quarterfinal against 2006 Australian Open finalist Marcos Baghdatis and said he’s feeling OK now.

Nadal, who has struggled with a sore left shoulder, was beaten in straight sets by Gael Monfils in the Qatar Open semifinals on Friday, but did not blame any injury problems for the loss.

There have been several other injury pullouts. Sixth-seeded Alex Bogomolov Jr. of Russia withdrew from Qatar with a right ankle injury. At Brisbane, Florian Mayer retired with a groin injury and Tommy Haas with a calf muscle injury.

Maria Sharapova withdrew before the Brisbane tournament with an injured ankle and Venus Williams announced in mid-December that she was pulling out of the ASB Classic in Auckland, New Zealand, which was supposed to be her first competitive match since August. The 31-year-old Williams is still recovering from the immune system disease Sjogren’s syndrome, which can cause fatigue and joint pain.

There has been no word on Venus’ status for the Australian Open. Her sister’s fitness for Melbourne may also be in doubt until just before the tournament starts.

“I’m going to take a couple of days off _ not too many _ and see how I feel,” Serena Williams, the 2010 Australian Open champion said after her withdrawal at Brisbane. “I’m still hopeful of playing the Australian Open.”