The Team Sky rider looked to be sailing along nicely at the front of a chasing bunch, but an intervention from the elements led to his bike taking an untimely detour with 58 kilometres remaining of the one-day classic.
He desperately tried to stay upright before leaping off his bike to avoid being hurled into a ditch.
The E3 Harelbeke winner clattered into the ground and broke into an unplanned roly-poly but, remarkably, a shaken Thomas quickly got back on his bike and re-joined the pack.
He even got himself back in contention for victory, only for Luca Paolini to launch a succesful attack in the final kilometres as the Italian veteran stunned the field to win in Belgium.
Thomas finished a credible third.