JUST a day after Nick Kyrgios told reporters if he’s playing his best it’s "tough to lose" at the Atlanta Open, the Australian has crashed out -- booed off the court as he retired hurt.

"I always feel comfortable playing here ... the court suits my game style," he said yesterday after a comfortable win over American Noah Rubin.

But that all came to a relatively abrupt end, as Kyrgios called it quits after 52 minutes, down 7-5, 3-0 in his quarter-final with 23-year-old Brit Cameron Norrie.

The Aussie was reportedly struggling with a hip problem and saw the trainer in the first set.

At times Kyrgios seemed disinterested in the contest with his effort in question late in the opening set before he meekly double-faulted to give Norrie the early break in the second.

Norrie addressed Kyrgios’ injury in his post-game interview.

"I really for the guy," the world number 73 said.

"He’s so talented. Who knows what he could do in tennis if he was healthy?"

Matt Ebden will fly the Australian flag in the last four after he required just 71 minutes to defeat Marcos Baghdatis.

The Australian world No.55 broke the Cypriot’s serve three times to secure a comfortable 6-3 6-2 win over the former Australian Open finalist. Ebden is through to his second tour-level semi-final of the year as he strives for his maiden ATP Tour title.

The 30-year-old has not dropped a set in Atlanta but will face a stern test next when he faces top seed John Isner for a place in the final.

The American was pushed to three sets by German sixth seed Mischa Zverev before he prevailed 7-5 4-6 6-1, dropped just two first-serve points in the match and sending down 21 aces.

Ebden is 1-3 against Isner but won their past meeting in January at the Australian Open.

"I am definitely playing better than I was in the beginning of the year. I was struggling a little bit," Isner, a four-times champion in Atlanta, said. "To his credit, he (Ebden) played much better than me that day in Australia. He had the home crowd on his side. It helps a lot.

"Now, I’m playing in my backyard and he beat me in his backyard. We’ll see if I can turn the tide." American eighth seed Ryan Harrison advanced to a semi-final clash with Norrie by beating South Korean third seed Chung Hyeon 6-7 (3-7) 6-2 7-6 (7-5). "This is exactly what the doctor ordered for me," Harrison said. "It was a little nervy from both of us there at the end."