Belgian Tom Boonen (Etixx-QuickStep) crashed and abandoned the Abu Dhabi Tour's second stage today near the Formula One Yas Marina Circuit.
The classics champion and former world champion fell after the immediate sprint at 55 kilometres. He came down with others including MTN-Qhubeka's Theo Bos and Songezo Jim.
Boonen, wearing the green points jersey, appeared to nurse his left side. After some time, he stood up and climbed into an ambulance.
Dutchman Bos also abandoned the 129-kilometre stage.
In Paris-Nice this year, Boonen crashed and dislocated his shoulder in the first stage of Paris-Nice on March 9. The incident forced him to miss the spring classics.
The 34 year-old - 35 next week - won the Tour of Flanders three times and Paris-Roubaix four times. In 2005, he won the world championships in Madrid.
The incident ends his 2015 season, but should not affect his 2016 season, which is expected to be his last as a professional cyclist.
Sky's Elia Viviani won the sprint, but attention turned quickly back to Boonen.
"He hit his head pretty heavily, he was a little bit slow responding to us," said race doctor, Stefano Tredici.
“He wanted to get up and get his bicycle, so we allowed him to do so slowly. Soon after, he decided not to go and we sent him to the hospital for checks.
"His helmet protected him a lot. He was dizzy, he told me he could not remember what happened, but he was answering to my questions, which was a good sign."
Tredici, based on the quick on-the-road check, said that Boonen suffered a head injury, but did not fracture any bones.
The doctor who treated Boonen said that he was unconscious for a few seconds, which was normal given the crash.
Bos's team-mate Jim hurt his hand, but continued.
"I was behind Theo Bos, Boonen was behind me," Jim said.
"Bos hit something on the road and fell, but I couldn't really see what happened. I went into him, and Boonen was also caught in the crash."
Etixx-QuickStep have since said Boonen will remain in hospital over night, but is concious and lucid, though suffered a left temporal bone fracture in the incident. Further examinations will take place tomorrow.
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