Milos Raonic crashes out of US Open against Ryan Harrison

Raonic won the opening set in a tie-break, but as the match progressed appeared to be struggling more and more with his fitness.

Harrison, meanwhile, was on his toes right to the end as he beat the fifth seed 6-7(4) 7-5 7-5 6-1, with Raonic barely running for anything by the end of the match.

"When you have the crowd behind you, like you guys were, you don't really realise how physical it is. You're just playing on adrenaline," said Harrison.

"He's a really great player, he played fantastic at Wimbledon... [but] I've found some form and I'm hoping to keep it going on Friday."

The mystery of what happened to Raonic is yet to be solved, but no doubt the Canadian - who entered the tournament with such high hopes - will explain what was ailing him when he comes in front of the world's tennis media.