It was painful to watch as Alexander Zverev's game completely fell apart at the start of his US Open semi-final against Pablo Carreno Busta.

And it was also painful to listen to as the commentators laid into his play as he slumped to a two-set deficit.

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"A quite diabolical performance in the second set from Sascha," said TV pundit Daniela Hantuchova.

"Such a lame first two sets - you're looking for a reaction right now!" said fellow panellist Tim Henman.

The German battled back to complete a win from two sets down for the first time in his career. He has been tipped for ages to be a contender to step up and challenge the old guard, and it feels like he's been a young pretender for too long.

In many ways, this has been a breakout year for him. He has reached a major semi-final for the first time in his career, but while his run to the last four in the Australian Open was hugely encouraging, his run in the US Open has left more questions than answers.

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Defeat to Dominic Thiem in Melbourne was by no means a disaster. He won the first set against the Austrian and could have reached the final had he performed better in clutch moments in two subsequent tiebreakers. Thiem has quietly become the man to challenge Novak Djokovic, Rafael Nadal and Roger Federer's hegemony and pushed the Serbian all the way in the final.

But here there have been some serious concerns about his style of tennis.

In his quarter-final against Borna Coric, he received as stark a warning as possible about the perils of starting slowly as he lost the first set 6-1. In the wake of the clash, Martina Navratilova accused him of playing "pat-a-cake" tennis, and received a prickly response from the 23-year-old.

"Maybe she should look at my record against the big guys. Maybe she should look that I'm positive against Roger. Maybe she should look that I've beaten Novak on multiple occasions in big matches and finals."

Patrick Mouratoglou, the coach who helped Serena Williams win a multitude of Grand Slams, has already warned Zverev about his need to change, and suggested taking a look at his second-serve tactics.

He has a great base, he moves incredibly well, he's very solid, but that's not enough."

Comparing him with Andy Murray, he added: "[He] was a fantastic player for a long time, but not a Grand Slam winner," Mouratoglou said: "Then he changed a few things in his game and became a winner, I think Sascha can do that, too, but he needs to evolve."

This is the core of Zverev's problems in Grand Slams. He can rightly point to big wins in big matches against the likes of Djokovic and Federer - but Djokovic and Federer themselves should serve as role models for him. They've constantly adapted to become the best and Zverev has not.

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He has the biggest opportunity of his life against Thiem in New York on Sunday - and he will certainly need to put in a much better performance than he has managed at any other time in the past fortnight.

And interestingly, much of the criticism about his performance on Arthur Ashe against Carreno Busta echoed criticism he had faced earlier in the week - and for months before.

"He should be going for two first serves," suggested Jim Courier. "He's hurting himself making fewer second serves than first serves...I would like to see him develop the kick-serve, right into the middle of the service box, and rely on his ground strokes to stay in the rallies.

"I think he needs a basic second serve, but that's not what he's been doing.

It's not easy - and it can be very painful to watch when he starts to suffer."

These people have no grudge against Zverev. They are saying these things because they know he can be better. It's frustrating and saddening to watch a player who could have such great potential fail on the biggest stages.

And when Jim Courier, Martina Navratilova, and Patrick Mouratoglou are all suggesting Zverev take a look at his second serve, he'd probably be best advised to do what they say.

After all, at the moment, they know infinitely more about winning a Grand Slam than he does. But that could change on Sunday.

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