The US Open challenge of Nick Kyrgios is in question after he was forced to retire during his Atlanta Open quarter-final with Cameron Norrie due to a hip injury.
With the final grand slam event of the year just a month away, world No.18 Kyrgios will have to overcome the recurrence of an issue that has plagued him since Queen's last year.
The Australian second seed was trailing British left-hander Norries 7-5 3-0 after 52 minutes of play when he decided he could not continue.
Kyrgios saw the trainer midway through the first set but that did not prevent the crowd booing him off court at the conclusion.
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.
The British world No.73 Norrie will play Ryan Harrison next but said he would have preferred to progress in different circumstances.
"It feels great to get into the semis but not the way I wanted to do it today," Norrie told the ATP's official website.
Matt Ebden will fly the Australian flag in the last four after he required just 71 minutes to defeat Marcos Baghdatis.
The 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 next faces top seed John Isner for a place in the final.
The American was pushed to three sets by German Mischa Zverev before he prevailed 7-5 4-6 6-1.
Ebden is 1-3 against Isner - a four-times champion in Atlanta - 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 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 Harrison advanced to the semi-finals by beating South Korean third seed Chung Hyeon 6-7 (3-7) 6-2 7-6 (7-5).
