OUDENAARDE, Belgium, April 2 (Reuters) - Philippe Gilbert became the first Belgian in five years to win a major classic after a gutsy attack with about 50 kilometres left earned him victory in a crash-hit Tour of Flanders on Sunday.
The Quick Step Floors rider, wearing the Belgian champion's black, yellow and red jersey, powered away in the cobbled ascent of the Oude Kwaremont and never looked back to clinch his third 'Monument' classic after titles in the Tour of Lombardy (2009, 10)and Liege-Bastogne-Liege (2011).
Pre-race favourites Peter Sagan, the world champion, and Greg van Avermaet, the Olympic champion, crashed with Oliver Naesen 17km from the finish when Slovakian Sagan hit a barrier on the side of the road.
Van Avermaet (BMC) finished second and Dutchman Niki Terpstra (Quick Step Floors) took third place.
Belgian Tom Boonen (Quick Step Floors), a three-times Tour of Flanders winner who will end his career in next Sunday's Paris-Roubaix, suffered a mechanical fault that dashed his hopes of a fourth title.
Boonen was the last Belgian to win a Monument when he claimed the Paris-Roubaix title in 2012.
Team Sky and Quick Step accelerated in the punishing cobbled ascent of the Kapelmuur some 95 km from the finish, splitting the peloton with Sagan, Van Avermaet and Degenkolb trapped behind.
Fourteen men -- including Boonen and Gilbert -- broke away and caught the eight early breakaway riders with 65 km left.
The main group came close at the foot of the Oude Kwaremont but Gilbert attacked in the cobbled ascent, 54km from the line, and built a 20-second advantage over the select group with the peloton at 50 seconds.
The pack reined in the fugitives with under 50km left with only Gilbert in front with a 40-second advantage.
Fabio Felline and Dylan van Baarle chased Gilbert, and defending champion Sagan jumped away from the main group in the ascent of the Taaienberg with four riders, including Van Avermaet, on his tail.
With 33 km left there were seven chasing Gilbert who had a one-minute lead.
Sagan accelerated in the last climb of the day, up the Paterberg, but his front wheel clipped a barrier and he took Belgians Van Avermaet and Naesen in his fall.
Van Avermaet, however, was quickly back on his bike to contest the sprint for second.
American Coryn Rivera upset the pre-race favourites to win the women's race in a tight sprint earlier on Sunday.
(Reporting by Julien Pretot; Editing by Clare Lovell)