Already 40 seconds down on the lead group, Cancellara and Sagan were pushing hard to connect back up with the group up the road, which formed nearly 100km earlier.
Cancellara, sitting third wheel in his group, slipped on a muddy part of the sector and went down hard, but Sagan, on the Swiss's wheel, somehow managed to avoid going down and continued his pursuit.
Cancellara managed to get back on his bike, but by then was nearly three minutes down on the front group and his race was over.
Both Sagan and Cancellara were stranded behind the Boonen group after a crash on one of the earlier cobbled sectors split the peloton.
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