Tsitsipas, who admitted afterwards he had been dealing with a right arm injury for some time, retired 2-4 down in the first set to Australia's Alexei Popyrin.

Following his withdrawal, the 23-year-old said: "I haven't retired once in my life, and it was something that I had to do today.

"I'm trying to be cautious for the next tournament, which is the most important one for me. 

"I have had an issue there for quite a while now.

"It has gotten bigger in the past couple of weeks, so I'm just trying to protect it. 

"I felt the pain playing in the match, and I just don't want for it to get worse than it is now."

Meanwhile, Briton Cameron Norrie continued his excellent run of form, besting Reilly Opelka of the United States 6-3, 6-4.

"I managed to put some returns in at the right times and managed to chip on my forehand," said Norrie.

"But more, I was just happy to get through at the end. 

"It was tricky."

This was Norrie's 50th Association of Tennis Professionals Tour victory this season, after winning just 51 in his entire career before 2021.

There were several shocks in the round of 32 stage.

Canada's number nine seed Félix Auger-Aliassime lost to Dominik Koepfer 3-6, 5-7, while Russia's number five seed Andrey Rublev lost to Taylor Fritz of the US, 7-5, 7-6.

Italy's Jannik Sinner, the number seven seed, lost 6-7, 5-7 to teenage sensation Carlos Alcaraz Garfia of Spain, and Pablo Carreño Busta of Spain, the number 12 seed, lost to home hero Hugo Gaston 7-6, 4-6, 5-7.

Germany's Alexander Zverev defeated Dušan Lajović of Serbia 6-3, 7-5.

The round of 16 gets underway tomorrow, with Serbia's world number one Novak Djokovic returning to action.