Nick Kyrgios withdrew from the Japan Open just minutes before his quarter-final on Friday with a sore left knee, calling it "heartbreaking".
The mercurial Australian star had been scheduled to play third-seeded American Taylor Fritz, who will now play Canada's Denis Shapovalov, seeded seventh, in the last four on Saturday.
At a hastily announced press conference when the match was due to begin, world number 20 Kyrgios said he had "been playing amazing tennis all year" but it had been physically taxing.
"I actually was dealing with a bit of a knee issue around the US Open time, and got back home, probably didn't take enough time off, to be honest. I went straight back into training," said the 27-year-old.
"It's heartbreaking. When the body lets you down, it's not a good feeling."
Kyrgios will now head home, but said he still plans to play several events in the run-up to the Australian Open in January, and promised to return to the Japanese tournament in 2023.
Kyrgios, who on Thursday beat Poland's Kamil Majchrzak 3-6, 6-2, 6-2 in the second round, said his knee had "not really" been an issue in his singles or doubles matches, but it had felt "pretty sore" while warming up on Friday.
"It's probably in this case something overloading, rather than not being fit enough," he explained.
"It's almost being too excited to get on the court, and maybe training a little bit too much."
Earlier on Friday, a jet-lagged Frances Tiafoe flew into the semi-finals in straight sets, clinching each with an ace to beat Serbia's Miomir Kecmanovic 6-0, 6-4.
The fourth-seeded US player will face unseeded Kwon Soon-woo of South Korea on Saturday.
Tiafoe, who beat Rafael Nadal at the US Open on the way to his first Grand Slam semi-final, said he had been "solid from the start" of the 71-minute quarter-final.
"Great tennis today," he said. "I was moving really well, I hit the ball really well from the back of the court."
The 24-year-old American, the world number 19, said he had been struggling to adapt to the Tokyo time zone.
"I've been really bad and jet-lagged, so I've probably been sleeping pretty early and waking up really early," he said.
Canada's Shapovalov won a place in the semi-final with a 6-4, 6-3 victory over Croatian Borna Coric, while Kwon beat Spain's Pedro Martinez 6-3, 6-0.
Tiafoe said it would be "cool" to play the South Korean, who he faced at the Tokyo Olympics.
"Kwon's a great player. He's been playing really well this week, so it's going to be cool to play him, we practise sometimes in Florida."
Tiafoe has been in the spotlight since stunning Nadal in September and playing a starring role at the Laver Cup in London, where his Rest of the World Team beat Europe for the first time in the competition's history.
He has said he hopes to inspire a new generation of Black tennis players.
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