Third seed Felix Auger-Aliassime said he was "dizzy and seeing spots" in a surprise second-round loss to Karen Khachanov at the US Open.

A two-time semi-finalist at the tournament, the Canadian lost 6-7 (5-7) 6-3 6-2 6-2 to the unseeded Russian.

Auger-Aliassime is the highest-ranked player to exit in the men's singles draw so far in New York.

The 26-year-old said afterwards he had started to feel unwell early on against Khachanov.

"After a set and a half, I just couldn't feel well and was dizzy and seeing spots and couldn't recover," he said. "I thought maybe it would pass, but it didn't.

"I've never felt quite like that. I've had cramps before. I've felt tired towards the end of matches, but in a normal way.

"This was abnormal a little bit, and there were no signs in the last couple of days and no signs this morning, as well."

Italy's 13th seed Lorenzo Musetti also struggled physically in the conditions in a 3-6 6-1 6-2 6-2 loss to Australian wildcard Dane Sweeny.

Temperatures reached 30C in New York on Thursday after several days of rain.

Their exits that of 24-time Grand Slam champion and fourth seed Novak Djokovic in the first round, with the Serb saying he "despised every moment I spent on the court for that".

Auger-Aliassime reached the quarter-finals of this year's Wimbledon and French Open, but withdrew from the Rogers Cup in Montreal in August with a back injury.

"I am not sure but maybe he had some physical issues and that is part of our sport," Khachanov said in his on-court interview.

"I wish him to feel better and speedy recovery if he is injured."