Melbourne:Rafael Nadal's injury-enforced withdrawal from the Australian Open has once again raised doubts as to whether tennis has seen the best of him.
The mercurial Spaniard limped out of his engrossing quarter-final on Tuesday with rising British star Andy Murray early in the third set, trailing by two sets, with a recurrence of a knee injury that blighted his 2009 season.
Nadal put a brave face on the disappointment of relinquishing his Australian title through injury and the likely imminent loss of his number two world ranking to either Murray or Novak Djokovic.
Yet it hardly augurs well for Nadal just weeks into the new season that questions are once again being asked about his long-term viability, and all incredibly at the age of 23.
Nadal was defensive when asked at his post-match press conference about whether his latest injury setback would force him to refine his physically-demanding game.
"Don't start, guys, with these questions right now. I think it's not the right moment," Nadal said.
The six-time Grand Slam champion contends that he has already changed his playing style from the early years of his meteoric career.
"I think I've changed my style of play," he said. "If you see my matches in 2004 or 2005, even 2006, I run a lot compared to now.
"I've just played against one of the best players of the world, and I think I wasn't running any more than him."
Nadal said he had been confident his injury demons of last year were behind him as he prepared for the Australian Open, but against Murray he wasn't going to risk further aggravating his troublesome right knee.
"I worked a lot and I didn't have lot of problems for the last six months," he said. "Against Murray is the first time I felt something after a bad movement.
"But it's not because the knee is tired. I think it's going to be okay."
Nadal said he was encouraged by his form at the year's opening Grand Slam and was leaving Australia feeling positive about his chances this year.
"I think my level was very high. I had big chances to win both sets (against Murray)," he said.
"I think I was very close against a player who is probably playing better right now.
"So I go out from this tournament very happy about my level because the perspective is much better right now than two months ago when I lost in the Masters Cup in Paris.
"In all those matches I didn't see any way of playing my best tennis again, but now I am playing at my best level."
Nadal has forged a reputation in just seven remarkable seasons of playing high-octane tennis.
He is a renowned spartan trainer, and has developed a powerful physique and awesome power with his left-handed groundstrokes.
But once again his body may be telling him something as injury again forces him to take a breather and recover.
Nadal was forced to withdraw from last year's Wimbledon where he was defending champion with tendinitis in both knees after his 31-match winning streak at the French Open was ended by Swede Robin Soderling in the fourth round.
Nadal was sidelined for nine weeks and after resuming suffered an abdominal injury and played with it at the US Open where he reached the semi-finals.
Now for the fourth Grand Slam running injuries have impacted on Nadal and the doubts start all over again.