Thanasi Kokkinakis is struggling to digest the most painful defeat of his burgeoning career after severe cramps cruelled his US Open campaign in New York.

The Australian teenage ace threatened to pull off a major upset before having to retire early in the fifth set of his first-round clash with Wimbledon semi-finalist Richard Gasquet on Tuesday.

Kokkinakis winced in pain as he was treated courtside and could barely move as brutally hot conditions claimed a raft of players at Flushing Meadows.

The 19-year-old gallantly tried to play on even after being reduced to under-arm serving before finally calling it quits after falling behind 2-0 in the deciding set and having the chair umpire advise him to consider his health.

After looking to the heavens and despairingly telling the tennis gods “it’s absolute torture”, Kokkinakis smashed his racquet into the net before staggering to the net to shake Gasquet’s hand.

“I’m pretty shattered,” he said after his 4-6 6-1 4-6 6-3 2-0 demise.

“It’s just frustrating ’cause I started playing good tennis. I was getting fired up.

“At one-all, 30-love in the fourth set, I was bouncing the ball. Then immediately I started to feel my forearm go. I’m like: `Oh, no, this is bad.’

“I didn’t feel tired physically either, which is the most frustrating thing about it. I actually felt all right.

“My energy levels were good, and then I went up to hit a swing and volley or something, and both my calves – it was just disaster.”

Kokkinakis has battled through several tough five-setters already in his career, but hadn’t suffered cramps since his first grand slam match as a 17-year-old.

“But I was starting to feel fatigued there,” he said.

“Like my whole body, I was starting to feel tired.

“This one, my body felt fine. Yeah, I was trying to push through. I hit the odd slap winner there, but I couldn’t sustain that and with a player like him, if you’re not 100 per cent, he’s going to move you around.

“I was dust.”

Kokkinakis plans to undergo tests to ensure he never succumbs to cramps again “because there’s nothing more frustrating”.

“I was playing good tennis. Would have been one of the biggest wins of my career,” he said.

“I’m pissed off more than anything.”

Kokkinakis will now turn his attention to Australia’s Davis Cup semi-final against Great Britain in Glasgow the week after the Open after he and Nick Kyrgios missed a place in the men’s doubles draw at Flushing Meadows.

“We missed the entry deadline,” Kokkinakis said.

“Everyone was saying we were in, but I knew myself we weren’t.”

Kokkinakis was anything but the only casualty in the punishing conditions, with 11 players failing to complete their first-round matches.

– AAP