As he enters Wimbledon as the No. 10 seed, Rafael Nadal just doesn't look right, mentally, according to a tennis expert. And he doesn't look right mentally because he doesn't look right physically.

Legendary tennis coach Nick Bollettieri is writing for The Independent during its coverage of Wimbledon this year, and he kicked off his column with an assessment of Nadal's game. The Spaniard and former No. 1 player in the world has been trying to rebuild his confidence and regain his physical presence since hurting his back and playing with myriad injuries in 2014.

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And while Bollettieri professes great admiration for Nadal, he just doesn't like what he sees in Nadal's current game.

"For years I always used to see the same expression when I looked at Rafa Nadal's face," Bollettieri writes. "It was hard to detect any emotions. He wore a look that simply said to his opponent: 'I've got you, buddy. You ain't goin' nowhere.'

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"When I look at Nadal today I see something different. I'm beginning to see signs of emotion in his face that never used to be there. I see signs of a lack of confidence. I see signs of self-doubt. And as soon as you start to have doubts, that's when you can be vulnerable.

"Nadal says he is physically fine and that his knees are not a problem, but I just look at him and wonder how healthy he is. I don't think his movement is what it used to be. I'm reluctant to pass judgment when I can't be 100 per cent certain how he is, but my own eyes suggest he might be struggling."

Bollettieri suggests that Nadal's of tennis that he has played since he was 16 has taken its toll - Nadal is a tennis version of a punch-drunk boxer.

I suspect it's just a question of age and miles on the clock," Bollettieri reports. "... Nadal has been one of the great defensive players, someone who wins points by grinding opponents down and making them play the extra ball."

Nadal told the Daily Mail that he has played a lot of golf to help take his mind off his struggles, but acknowledges the considerable task ahead. He talked about the feeling he had when playing on clay as opposed to playing on grass.

"When I was playing at my best, I felt on clay as if I had time for everything," Nadal said. "I could play with different styles, aggressive, more defensive. That makes me feel under control almost all the time.

"On grass, it's a different story. On grass, the feeling is that you have to be very focused on every point. You cannot have two mistakes in a row with your serve because that means you've almost lost the set."

And remember, he lost to Novak Djokovic in the quarterfinals of the French Open on clay, his favorite surface.