Alexander Zverev reached his first grand slam semi-final with a four-set victory over Stan Wawrinka at the Australian Open.

The 22-year-old German went into the clash having not dropped a set and recovered from a poor start to record a 1-6 6-3 6-4 6-2 victory.

Having been criticised for under-performing at the slams, there is a certain irony that Zverev's big breakthrough has come at a tournament where expectations were probably at their lowest.

Zverev had a difficult 2019 and arrived in Melbourne with his serve apparently in crisis and having lost all three of his matches playing for Germany at the ATP Cup.

But his serve has clicked back into gear here and he has shown the sort of efficiency that had proved elusive at the biggest tournaments.

He won just 11 points in the first set against 2014 champion Wawrinka, who felt his quarter-final victory over Daniil Medvedev was the best he had played since knee surgery two and a half years ago.

That came over five gruelling sets and this was perhaps a match too far, with the veteran Swiss unable to sustain the pace as Zverev moved through to a semi-final clash against either Rafael Nadal or Dominic Thiem.

The seventh seed is now only two victories away from being able to fulfil his promise to donate all his prize money to the wildfire relief efforts should he go on to lift the trophy. 

"It feels awesome. I've done well in other tournaments, won Masters, won World Tour Finals but I never could break that barrier at a grand slam," said Zverev.

"You guys cannot imagine what this means to me and I hope this can be the first of many."

The German promised to donate the entire prize cheque of 4.12 million Australian dollarsto the wildfire relief efforts should he win the title.

That seemed an unlikely occurrence at the start of the tournament, and Zverev said with a chuckle: "Easy to say in the first round, right? Yes it is still true. I hope I can make it happen. I made the people of Australia a promise. I will keep that promise if it happens."