World number one Novak Djokovic quit the Dubai Tennis Championships after dropping the first set of his quarter-final against Feliciano Lopez on Thursday, ending his stunning streak of 17 successive finals.
The top-seeded Serb lost the opener 6-3 during which he appeared to complain about a problem with his right eye.
The retirement represents the first time the 28-year-old has failed to make a final since Doha in January 2015.
"It's very sad when one of the greatest players ever has to retire from a match like that," Lopez told reporters while adding that Djokovic had told him he was suffering from an eye infection.
At one point, Djokovic asked for the doctor to come on court, but refused the eye drops offered to him.
He committed 18 unforced errors in the nine games played, offering his hand to his Spanish rival after just half an hour on court.
Australian Open champion Djokovic, who racked up his 700th career win on Wednesday, was booed by some fans as he walked off court having suffered his first loss in 15 matches in 2016.
French Open champion Stan Wawrinka reached the semi-finals where he will face brash Australian Nick Kyrgios for the first time since their infamous duel in Canada last year.
Wawrinka saw off Germany's Philipp Kohlschreiber 7-5, 6-1 while Kyrgios stunned third-seeded Tomas Berdych of the Czech Republic 6-4, 6-4.
When the pair met in Montreal last August, Kyrgios was widely lambasted for making crude sexual remarks on-court concerning Wawrinka's girlfriend Donna Vekic, a Croatian WTA professional.
Kyrgios was handed a suspended 28-day ban and $25,000 fine.
But the Australian, ranked 33rd and who won his maiden title in Marseille last weekend, is expecting no fireworks on Friday.
"Since the incident. I think we've both moved on from it," said Kyrgios.
"We played on the same team in the IPTL (International Premier Tennis League). That was actually really good. I got to know him a little bit more.
"He played great there. He's one of the best players in the world, no doubt. He's playing great tennis."
Second seed Wawrinka had to break in the final game of the first set against Kohlschreiber after twice losing leads.
He then took hold of the match to clinch a runaway victory with 27 winners and five breaks of serve.
Kyrgios brought out the physio after the fifth game of his opening set against Berdych for a tight back.
The right-hander, who was injured during the Australian Open in January, has been going flat-out since returning last week.
"I wasn't in the greatest shape, obviously. I think I got a bit of food poisoning last night," said Kyrgios who had also defeated Berdych in Marseille last week.
"I was really surprised with the performance I gave out there today, coming out and beating a quality player and in tough conditions.
"It's probably my favourite win over the last couple of weeks."
He will be tested physically even further next week with a Davis Cup tie in Melbourne against the United States.
Berdych, a two-time Dubai finalist, had reached the semi-finals at the Aviation Club in four of the last five years.
Kyrgios took 41 minutes to win the first set but dominated in the second, losing only three points on serve as he rolled to the win, finishing with a 10th ace.
Marcos Baghdatis returned to the semi-finals after a six-year absence as he knocked out fourth seed Roberto Bautista Agut 7-5, 6-0.
The first set was a 67-minute struggle while the popular Cypriot broke it open in the second to send the Spaniard out.
Baghdatis overcame four double-faults in the eighth game of the first set to suddenly turn his momentum.
"That was the most important game of the match -- and the week," said the 2006 Australian Open finalist who faces Lopez on Friday.
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