Lleyton Hewitt has hit back at John McEnroe and revealed Nick Kyrgios may well have quit the US Open much earlier had his third-round match not been played on tennis’s biggest stage.

Meanwhile, Kyrgios has lashed out at Hewitt’s former coach Roger Rasheed for his swipe at the world number 16.

Former tennis great and now commentator McEnroe challenged Kyrgios to quit the sport if his heart wasn’t in it before the 21-year-old retired with a hip injury while trailing Illya Marchenko two sets to one on Saturday night (Sunday AEST).

But Hewitt, who is sweating on Kyrgios being fit for Australia’s Davis Cup playoff with Slovakia in Sydney next week, says McEnroe is being too harsh.

“It’s unfair,” the Australian captain told AAP on Monday.

“He was obviously playing with an injury and I know before the first round that he did have a small hip problem that he actually did two days before the tournament in a practice session.

“It was obviously causing him some discomfort in the first couple of matches. He called the trainer out and he was feeling it.

“He probably would have pulled out sooner in that match last night apart from (the fact) he was on centre court, a big match on Arthur Ashe Stadium and he was kind of the name player in that match as well.

“For him to get criticised like that, he would be pretty frustrated.”

Hewitt will name Kyrgios in Australia’s four-man team on Tuesday, but knows from his own experiences with hip flexor injuries that the world No.16 is only a 50-50 chance of playing in the September 16-18 tie.

John McEnroe blasted Kyrgios for pulling out of the US Open. Photo: Getty

“We’ll give him every chance to obviously get right and be fully fit,” Hewitt said.

“We probably won’t know a lot, though, for a few days until he gets back to Australia and really how it settles down and then we’ve got to weigh up whether he could maybe make the injury worse and weigh up when he can start practising.

“Then we’ll have to make that decision closer to the tie whether he’s going to get enough hitting on grass to be able to go out there and do a job for us, or if it’s too big a risk.

“It’s obviously not the best situation for us, but he’ll be named.”

Kyrgios is pledging to learn from his bitterly disappointing Open experience.

“This one hurts man,” he posted on Twitter as he struggled to digest his first defeat in 21 matches against an unseeded player at a grand slam.

This one hurts man… My sights were set on some great things this week. Goodluck to everyone still going ✌🏾️ pic.twitter.com/X0ikxzz2l1

— Nicholas Kyrgios (@NickKyrgios) September 4, 2016

Not the way I wanted to end the last grand slam of the year. F**king sucks. #liveandlearn https://t.co/d8JeoVm682

— Nicholas Kyrgios (@NickKyrgios) September 4, 2016

Its only a failure if you don't learn the lesson. #flushingmeadows16 https://t.co/jMgq4w1PXr

— Nicholas Kyrgios (@NickKyrgios) September 4, 2016

And while Kyrgios said he was prepared to take criticism from seven-time major winner McEnroe, his reaction to Rasheed’s comments was not so friendly.

https://twitter.com/NickKyrgios/status/772891911094874113?ref_src=twsrc%5Etfw

Rasheed has been critical of Kyrgios and young compatriot Bernard Tomic in the past.

He has not been entirely against the idea of coaching Kyrgios in the past.

“If a player with a high skill set is willing to commit and wants to be a [top] player in the game, my coaching process delivers,” Rasheed told Fairfax Media in 2015.

“You’ve got to be willing to buy into that, buy into actually wanting to be a great player.

“Obviously Nick’s a different character. He’s flamboyant, and you’re never going to take away the natural flair of the athlete. But it’s about how you can actually bring that and mould that into a product that can legitimately … be a force on the world stage.”

Rasheed’s highest ranking as a player was 134.

-AAP