Wimbledon 2015: Nick Bollettieri - You can see the doubt in Rafa Nadal's face these days

I worry that the former champion is struggling physically and that his unrelenting playing style may finally be catching up with him

Rafa Nadal is showing signs of self-doubt after victories have been hard to come by this year (Getty)

For years I always used to see the same expression when I looked at Rafa Nadal’s face. 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.”

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 judgement when I can’t be 100 per cent certain how he is, but my own eyes suggest he might be struggling.

It always used to be the case that the longer rallies went on, the greater the advantage to Nadal. His attitude towards his opponent used to be very simple: “Go on, amigo. If you hit 10 shots, then I’m going to hit 11. If you want to hit 22 shots then I’ll be happy to hit 23.”

That’s not what I see today – and I think other players are starting to see a change too. They detect that he’s not quite the player he used to be. The long rallies don’t necessarily favour him any more.

I suspect it’s just a question of age and miles on the clock. Nadal turned 29 earlier this month. He’s been playing on the main tour since he was 16 and he’s played nearly 900 matches.

You can point to the fact that Roger Federer is still going strong, even though he’s nearly five years older and has played more than 1,200 matches, but you have to consider the way the two men play the game. Federer attacks the ball and shortens points. Nadal has been one of the great defensive players, someone who wins points by grinding opponents down and making them play the extra ball.

Nevertheless, he’s still someone you wouldn’t rule out of the equation. Although he’s had some bad defeats at the All England Club in the last few years, losing to Lukas Rosol, Steve Darcis and Nick Kyrgios, you don’t get to five Wimbledon finals without being a master of grass-court tennis.

I’m not sure either that losing to Novak Djokovic at the French Open will have that big an effect on him mentally. As far as Rafa is concerned that will be history.

What I do worry about is how he would fare if he came up against the best players, how he would cope with a Federer or a Djokovic or an Andy Murray. I think it would be tough for him to beat any of those guys.

I hope I’m wrong because Nadal’s a great person. If we had more people like him in the world it would be a much better, richer, healthier and more gentle place.

So who can win the men’s title? You would have to rate Djokovic and Murray as the two favourites. I think there will be a lot of pressure on Djokovic as the defending champion, but he is having a great year. I’ve been very impressed with Murray too. It took him time to recover after back surgery, but he’s playing some great tennis again. That win at Queen’s Club last weekend won’t have done him any harm at all.

Can Federer win an eighth Wimbledon title? Boy, stranger things have happened. He has improved his serve, especially the wide slice. Playing with a bigger racket has clearly helped him and I like the way his coach has encouraged him to move closer to the baseline. Now I think the key is to get into the net as much as possible. The more Federer gets into the net, the better his chances.

Having said all that, I think you can never discount the spoilers, the guys who are capable on their day of surprising the top players. The so-called “Big Four” have dominated the sport for 10 years, but they are all getting older and I see plenty of threats out there. The top guys just don’t have the easy passages in the first couple of rounds that some of their predecessors had.

Nick Bollettieri is one of the world’s most admired tennis coaches whose players have included Andre Agassi, the Williams sisters and Maria Sharapova. He will be writing for The Independent on Sunday, The Independent and the i throughout Wimbledon

A difficult year: How Nadal has fared in 2015

Date/Tournament/Round exited/Beaten by

Jan 6 Qatar Open/First/Michael Berrer (Ger)

Jan 27 Australian Open/QF/Tomas Berdych (Cz)

Feb 21 Rio Open/SF/Fabio Fognini (It)

Feb 28 Argentina Open/WON [Juan Monaco (Arg)]

Mar 20 Paribas Open/QF/Milos Raonic (Ser)

Mar 29 Miami Open/Third/Fernando Verdasco (Sp)

Apr 18 Monte-C Masters/SF/Novak Djokovic (Ser)

Apr 23 Barcelona Open/Third/Fabio Fognini (It)

May 10 Madrid Open/Final/Andy Murray (GB)

May 15 Italian Open/QF/Stan Wawrinka (Swit)

Jun 3 French Open/QF/Novak Djokovic (Ser)

Jun 14 Mercedes Open/WON [Viktor Troicki (Ser)]

Jun 16 AEGON Champ/First/Alexander Dolgopplov (Ukr)