John McEnroe has challenged Nick Kyrgios to quit professional tennis if his heart is not in it.

Commentating on American television, McEnroe was scathing of Kyrgios's demeanour before the Australian retired from his third-round US Open match against Illya Marchenko with a hip injury.

Kyrgios was trailing two sets to one - 4-6, 6-4, 6-1 - when he called it quits.

The 14th seed complained of the injury during his first two matches and said he felt like he'd been "hit by a bus" at one point during Sunday's (AEST) defeat.

McEnroe has been critical and complimentary of Kyrgios in equal measures since the 21-year-old's emergence as one of tennis's hottest prospects.

McEnroe has both praised and criticised Kyrgios over his career. ( Getty Images: Jordan Mansfield )

Last year he lashed Kyrgios for his "bonehead" tactics during a first-round loss to Andy Murray at Flushing Meadows, while he also had cracks at the world number 16 at Wimbledon.

But his criticism of Kyrgios on Saturday night could be considered harsh, given the youngster's obvious injury concerns.

"Nick Kyrgios, you don't want to be a professional tennis player, do something else," McEnroe said.

There was no sign of any trouble early as Kyrgios broke Marchenko on the Ukraine's first service game.

He then held from 15-40 to consolidate for a 3-0 lead, the only break he needed to secure the opening set.

At times he toyed with the world number 63, producing a series of nonchalant backhand-down-the-line winners to appear in cruise control.

Kyrgios lit up Arthur Ashe Stadium when he ran down a drop shot and conjured an incredible around-the-net-post backhand winner in the fifth game of the third set.

Out of nowhere, though, Kyrgios dropped serve at 4-4 after a double-fault and forehand miss to hand Marchenko the chance to put the match back on level terms.

He made no mistake as Kyrgios's movement began to diminish.

After all but conceding the third set in 34 minutes, two courtside treatments from the trainer didn't help as Kyrgios repeatedly moaned: "I can't move. I can't turn, mate. What's the f***ing point?"

Marchenko sympathetic

Marchenko admitted it was not the way he wanted to reach the last 16 of a slam for the first time, five days shy of his 29th birthday.

"I feel sorry for Nick," he said.

"But it's a dream come true. A win is a win and a win here on Ashe is a very special feeling for me.

"I cannot explain how happy I am."

The Ukraine will play two-time grand slam champion and third seed Stan Wawrinka on Monday for a place in the quarter-finals.

The loss was Kyrgios's first in 21 matches against unseeded opponents at grand slam events and will come as a bitter disappointment for the 21-year-old who had high hopes at the Open.

Despite the setback, Kyrgios is set to climb to a career-high No.15 in the world after the tournament.

AAP