Team INEOS says Thomas had to drop out of the fourth stage of the nine-day Tour de Suisse race in Switzerland.
The team says on Twitter that Thomas “was alert and speaking to the team after the crash and will be taken to hospital for checks.”
Thomas had a ripped jersey and cuts on his back and shoulder after hitting the road with about 18 miles left in the stage. He was in eighth place, 28 seconds behind race leader Peter Sagan.
Thomas won the 2018 Tour after Froome won four times between 2013 and 2017.