The Trek-Segafredo rider was crashed in the first three hours of the 200-mile (321.9km) gravel race, which kicked off at 6:00 local time in Emporia, Kansas. Simmons, 21, was not present at the first time-check after 68.3 miles of racing.
His team confirmed that Simmons had left the race, noting that he had been chasing back after suffering a puncture when he crashed. He suffered a knee injury which required stitches as a result of the fall.
"Unfortunately, Quinn Simmons is out of the race due to a crash, while he was chasing back after a flat tyre," read the tweet issued by Trek-Segafredo. "He's now on the way [to] hospital for stitches to his knee."
For Simmons, who earlier this year returned to racing after being suspended by his team for 'divisive, incendiary, and detrimental' statements on social media, the event represented something of a return to his roots considering his background in mountain biking and second place at the 2019 Leadville 100.
At the 99.4-mile (159.9km) mark of Unbound Gravel, a group of five male riders led the race, having reached the checkpoint after four hours and 52 minutes of racing. Former WorldTour pros Peter Stetina, Laurens ten Dam, Ted King and Ian Boswell were all in the lead group along with 2019 winner Colin Strickland.
Three further riders had made the checkpoint in under five hours, including Rally's Robin Carpenter, while Flavia Oliveira and Lauren De Crescenzo were the quickest women after 68.3 miles, clocking times of three hours and 20 minutes.
The first finishers of the Unbound Gravel 200 are expected to cross the line at 16:00 local time (22:00 BST).
Update from @unboundgravel 👇Unfortunately @QuinnSimmons9 is out of the race due to a crash, while he was chasing back after a flat tire. He’s now on the way hospital for stitches to his knee 🤕June 5, 2021
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