Stan Wawrinka concerns me. I don’t know what’s going on in his head, but when I’ve watched him lately I’ve sensed that he’s not happy on the court. He seems to have lost confidence. You see his head drop when things start to go against him. He gets easily frustrated.

Holy mackerel, that isn’t the Stan we’ve all come to admire in the last two or three years. That isn’t the Stan who won his first Grand Slam title at the Australian Open in 2014. And it certainly isn’t the Stan who threw caution to the wind to blast Novak Djokovic off the court in last year’s French Open final. Boy, that was the performance of a player who was bursting with confidence.

It wouldn’t surprise me if Stan has trouble in his second-round match here against Juan Martin del Potro. Even though he has had to adapt his game after wrist surgery, the big Argentinian still has weapons that can damage you.

This is the only Grand Slam tournament where Wawrinka hasn’t made the semi-finals or better, but there’s no reason why he shouldn’t do well here. He has a big serve, a thumping forehand and a lovely single-handed backhand. When he’s cooking, you can get burned.

I think the result will all be down to Wawrinka’s mental state because if his head isn’t right he could easily lose. However, he shouldn’t be thinking that way because in theory Del Potro is there for the taking.

When he won the 2009 US Open Del Potro had the world at his feet, but he’s never been the same since he first started having wrist trouble not long afterwards. Without those problems I’m convinced he would have won more Grand Slam titles by now.

An injury like that can mess with your head. Hell, every time you hit the ball you might be wondering whether your wrist is going to go again.

Del Potro had one of the most feared double-handed backhands in the business, but these days he usually plays slice. And when you slice so much, it’s inevitable that at some stage you’re going to hit the ball short. Do that and you’re toast. <p>Nick Bollettieri</p>

On the two-handed backhand it’s the top hand that is the boss. The bottom hand just holds the racket. If you can’t use that top hand properly you’re in trouble. It’s very difficult to control the ball or to hit it with any power.

Del Potro had one of the most feared double-handed backhands in the business, but these days he usually plays slice. And when you slice so much, it’s inevitable that at some stage you’re going to hit the ball short. Do that and you’re toast.

Nevertheless, the big man still has a huge serve and a thumping forehand that can cause plenty of damage. If he has a big-serving day and can bring his forehand into play, he could just spring a surprise.

Raining champions

We’ve all got used to rain delays at Wimbledon over the years, but the frustration never gets any easier. It’s hard enough waiting in vain for a match to start, but it can be even tougher if there are interruptions.

Rain breaks are usually most difficult for the player who is winning. If you’re in front, you just want to get on with it. Instead you have to be very strong mentally and tell yourself that the weather is exactly the same for both players. It’s all about how you cope with that.

Coaches can play a big part in rain breaks. My policy was usually to talk very little about the match during any breaks. I would just sit down with my player and maybe play some cards or backgammon. Often we wouldn’t even say a word about the match.

Rain has plagued the championship so far (Getty)

When you’re losing, a little bit of advice from the coach can make all the difference. In those circumstances I used to give my player just a little tip here or there. If you start talking about technique or anything like that, you can kiss the result goodbye.

All players are different. Some need calming down, others just need to get themselves in the right frame of mind to play again. Andre Agassi never liked to talk too much. Aaron Krickstein used to play cards. Boris Becker didn’t say too much, but he did like me to give him little tips. He was the same before a match. He used to say: “Mr B, just tell me one little thing.”

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If rain days are tough for the players I also think we should consider what it’s like for the fans. They’ve paid good money for their tickets and maybe taken days off work. Some of them will have queued all night. The next time a player complains about rain breaks maybe they should be reminded what it’s like for the fans out there.

Who I met today

Great to see Tommy Haas, who has been at the IMG Academy in Florida for more than two decades. Tommy has had so much bad luck with injuries that I’d never buy a Lottery ticket off him. He has had five major operations but at the age of 38 he wants to give it one more try. He’s hoping to come back and play at next year’s Australian Open.

I was delighted to see Tatiana Golovin. She’s a lovely girl. I remember when Tatiana, Maria Sharapova and Jelena Jankovic were students together at the academy. When they were teenagers it would have been hard to pick between them. Tatiana, unfortunately, was never the same again as a player after going down with a blood disease.