By Tracy Austin Two-time US Open champion

The shock Ana Ivanovic defeat to Jie Zheng did not surprise me as much as Maria Sharapova's on Thursday.

As world number one you would expect Ivanovic to beat Jie Zheng, even if she is having a bad day. There are matches when your timing is off a bit but champions somehow get through.

The reason that I am not surprised by the loss is that Ivanovic has struggled with pressure throughout her career.

Ivanovic cuts a dejected figure during her surprise defeat

Some players, when they are 14 or 16, have already got it, they have that mental toughness and have no trouble competing when a match is poised at 4-4 and there is a lot on the line.

We saw Ivanovic in the final of the French Open last year and she got overtaken by nerves.

Since then she has got fitter and slowly put each piece of the puzzle into her game but this is the first Grand Slam when she has been coming in as world number one as well as the number one seed.

She won the French Open only two weeks ago and it is a lot to take in.

All of a sudden you are the hunted, not the hunter. Everybody is after you. It takes some champions a little bit more time to adjust to each situation.

I saw enough of the match at the end to see that she was making some wild shots and she looked a bit uncomfortable.

That was down to the pressure. She goes from being tentative to going for big shots when completely off balance.

Justine Henin was a similar player. She did not come on the tour at 15 and handle the pressure beautifully. She had to learn to grow into it and deal with the expectations.

That's something that Ivanovic is going to have to talk to her coach about. How can I adjust when I am feeling totally out of sorts? How do I deal with those expectations?

She has got to have a different mind-set.

By John McEnroe Three-time Wimbledon champion

Jie Zheng must have seen that Ivanovic was not on her game after watching the game in the last round where she saved match points.

In that situation, one of two things happen. The player continues to struggle or they step it up two or three notches because she has that ability having won the French Open.

Over the last year we have seen Ivanovic go through a learning process.

From getting overwhelmed by Justine Henin in last year's French Open final, then doing better at the Australian Open with a more respectable performance in the final to winning the French Open at the start of the month.

At this year's Wimbledon you did not see that smile and exuberance about being at a big event. She started to feel the weight of the world on her shoulders and the expectations of being the world number one. It seemed to get to her.

I would say she found adapting from the clay of Paris to the grass of Wimbledon a bit harder than she expected.

In wet conditions like we saw on Friday, the ball does have the tendency to move through the court and that played to Jie Zheng's strength.

Jie Zheng's win shows that there is more depth in the women's game now. That's good to see.

She has had a lot of injury problems, she could not play here last year and it's nice to see players that you don't hear about capitalising on opportunities.

And with Sharapova losing yesterday and now Ivanovic, that's going to make a lot more players believe on both the men's and women's side.

Tracy Austin was talking on BBC Sport's Wimbledon coverage; John McEnroe was speaking on BBC Two's Today at Wimbledon.