Rafael Nadal has an argument as the toughest athlete alive, in any sport.

Sports Illustrated was one of several outlets to report Monday that the 14-time Grand Slam Champion, already out after having an appendectomy, now will undergo stem cell treatment on a back that he injured 10 months ago at the Australian Open.

Nadal could be opting for this procedure because he already had the treatment for his ailing knee last year. Injecting stem cells into the point of pain is designed to regenerate cells and create an anti-inflammatory effect, the Guardian added.

Nadal, who lost a five-set final to Stanislas Wawrinka in the Australian Open, also sat out part of the season with a wrist injury.

Rafael Nadal determined to play ATP World Tour Finals despite appendicitis

The back issue could help explain Nadal's inconsistent clay-court season, in which he lost three times prior to the French Open.

But Nadal came back and won his ninth French Open title, bad back and all.

A wrist injury sidelined him for three months, including his date at the U.S. Open. SI asked his uncle and coach, Toni Nadal, why Nadal didn't take off that time to try to have surgery.

"He's had them (stem cells) for the past six months," said Toni Nadal. "It's stem cell treatment, and doing it properly requires five weeks," he added when explaining why they did not take advantage of the three months between July and October. "Because we all thought that he'd play at the U.S. Open, we were more focused on his wrist injury, which looked to be healing well."

"I'm not saying it was completely down to his back, but it's true that he wasn't fit to compete to the best of his ability," he added.

If the time off from the wrist injury did not help his back, that could be an indication of how serious he back injury is.

Nadal attempted to play some tournaments after the U.S. Open so he would be ready to play in the ATP World Tour Finals that are currently going on. But then he revealed that he was suffering from appendicitis and had to withdraw.

Nadal, however, did not withdraw because he had appendicitis; he withdrew because the illness was sapping his strength and energy, leaving him more vulnerable to losing. He said he was not competing at the level he expected of himself.

Still, with the bad back, Nadal won four tournaments on the season.

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