Alexei Popyrin thought he was “done” deep in the third set as cramps set in before rallying to record the best and biggest win of the Australian’s career with a three-set triumph over grand slam champion Stan Wawrinka to win the Croatian Open.

Popyrin, 23, had to rally from a set down, declaring he managed to “dig deep” and find the will to take down Wawrinka 6-7 (5-7) 6-3 6-4 in a match that nearly lasted three hours in Umag.

“All tournament I’ve been fighting battles that I don’t know how I won. This one tops it all,” Popyrin said after claiming his second ATP title.

“My quad was done at 2-1. I felt it cramp, I felt it strain. I think it’s pulled.

“But I managed to dig deep. I don‘t know how I won it, honestly. I honestly don’t know.

“I even told the physio when he came out. He said, ‘If you go any more then it will be tough’.

“I said, ‘There’s no point in quitting now, I’m here in the final. If I lose, I lose. If I retire, I lose anyway, so I have a better chance of winning if I stay in’.

“So that‘s what I decided to do.”

Popyrin, from Sydney, joins the likes of Wimbledon champ Carlos Alcaraz, Wawrinka and a series of former world No.1s as Croatian Open champion.

He said the enormity of taking down three-time grand slam champion Wawrinka was not lost on him.

“It‘s an unbelievable honour to play somebody like Stan in a final,” Popyrin said.

“I grew up watching him, Roger, Rafa, Novak play. To play him in a final and beat him in a final, words can‘t describe how happy I am. It’s an unbelievable feeling.”