Angelique Kerber showed why she cannot be considered the favourite in victory over Shelby Rogers

For Nick Bollettieri, the answer comes down to what goes on inside Kerber’s head

Angelique Kerber survived a major scare to reach the fourth round (Getty)

Angelique Kerber is the world No 1 and was Wimbledon runner-up last year. Serena Williams, who beat her in the final 12 months ago, is not defending her title. Why the hell, you may ask, does nobody ever mention the 29-year-old German when they talk about potential champions next week?

For me the answer comes down to what goes on inside Kerber’s head. When things start to go against her she can lose her poise and get flustered. To be a champion you have to know how to handle adversity and get through periods when things are going against you.

Kerber is through to the fourth round thanks to her victory over Shelby Rogers, but there were times when the American was hitting her off the court. Kerber’s second serve is a particular weakness and one which all her opponents can target.

The longer the rallies go on the better it suits Kerber because she’s a fine athlete who gets a lot of balls back. However, I think she’ll be vulnerable when she comes up against someone who is able to put her under more sustained pressure.

That could happen as soon as Monday when she faces Garbine Muguruza, who is really finding some form again and looked in great shape against Sorana Cirstea. I like the way Muguruza hits the ball early and takes immediate charge of the points.

In some respects she reminds me of Maria Sharapova, though Muguruza’s strokes are more fluent and she’s also a better runner.

Monday’s match of the day

Simona Halep’s meeting with Victoria Azarenka promises to be one of the matches of the tournament. Among the women nobody has impressed me more this week than Halep. Holy cow, what a player she is. She’s quite small by current standards but she’s a great ball striker and an excellent athlete.

For me the biggest question mark over Halep is whether she has it in her to win a Grand Slam title. She’s been runner-up at the French Open twice and in both finals let winning positions slip from her grasp.

Azarenka is looking very good considering that she has only just returned after a year out of the game. Motherhood clearly suits her. She’s been striking the ball well on both sides and has been moving really beautifully.

However, Azarenka could easily have lost to Heather Watson in her last match. In the end I think the emotion of the occasion got to Heather and she wasn’t quite able to handle it. That’s where Azarenka’s big-match experience made a difference.

Tales from the IMG Academy

People often ask me who was the most talented player I ever coached at the academy. That’s a tough one considering that I’ve coached 10 world No 1s, but my answer is always the same: Marcelo Rios.

Rios never won a Grand Slam title, but, boy, did he have talent. Being a leftie gave him an advantage and he was a wonderful shot-maker. I loved the jumping two-handed backhands he could hit from almost anywhere on the court with both feet off the ground.

Marcelo was also a tireless worker, but he never achieved what he should have done because there were major fault lines in his character. He had no respect for other players, for spectators or for the traditions of tennis. He did nothing to help promote the sport in his home country of Chile and was very mean with his money.

You might think those faults would have nothing to do with his tennis, but I believe they were evidence of some inner weaknesses which stopped Marcelo reaching the heights that he should have done.

A tip from the top

I have always believed in the power of positive thought. If you think you’re not a good player and if you think you can’t hit a winner, those negative thoughts will lower your chances of winning. Be positive, man! If you go out there believing in yourself you’re so much more likely to succeed.

The perfect tennis player

Each day during Wimbledon I’m building a picture of the ideal tennis player by looking at a different aspect of the game. Yesterday I looked at the one-handed backhand, so let’s consider the two-handers today.

Looking back, both Andre Agassi and Jimmy Connors had great attacking two-handed backhands, though I don’t think anybody was as relentlessly consistent with that stroke as Bjorn Borg. Amongst the women, Chris Evert and Martina Hingis were also very reliable on the backhand side.

Today I have great admiration for the two-handed backhands of Maria Sharapova, Serena Williams, Novak Djokovic, Andy Murray and Kei Nishikori. Of those, I think I probably have to give Djokovic the edge because he so rarely misses on that side. The best of all time? I find it impossible to choose between Djokovic and Agassi.