He's nowon course for a record-equalling sixth green jersey.
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World champion Peter Sagan powered to his second stage win at the Tour in a perfectly-timed burst of speed.
He's now extended his lead in the points classification and is on course for a record-equalling sixth green jersey.
Italian Sonny Colbrelli was second and Belgian Philippe Gilbert third.
Another Belgian, Greg van Avermaet, clung on to the overall leader's yellow jersey.
The peloton dozed off during the major part of the 194.5km ride from Lorient, only controlling a seven-man breakaway that ended about 10km from the line.
All the overall contenders finished safely with the main bunch.
Thursday's sixth stage has a similar profile but a harder uphill finish in Mur de Bretagne where Sagan will probably struggle to take a third stage win.
France's Alaphilippe, who is six seconds off the pace overall, is widely expected to take the yellow jersey as the finale perfectly suits his abilities.