Remco Evenepoel won the San Sebastian Classic on Saturday in a thrilling race that also saw Julian Alaphilippe compound his Tour de France disappointment by dropping out less than halfway through.

The fresh-faced Belgian becomes the youngest ever winner of a World Tour race at 19 years and 190 days of age, and added to his Tour of Belgium win in June and last week's victory in Aalst after stretching away from the peloton with 20 kilometres to go and never looking back.

Deceuninck-Quick Step rider Evenepoel is in his first year as a professional rider but has already been compared to countryman and cycling legend Eddy Merckx, and he looked in prime form as he crossed the line 38 seconds ahead of the chasing pack.

Evenepoel's teammate Alaphilippe, who held the yellow jersey at the Tour de France for 14 stages before being overhauled by eventual winner Egan Bernal, was forced to abandon the Basque Country race 90 kilometres into his ride.

The 27-year-old Frenchman reportedly pulled up with muscle fatigue built up since the end of the Tour, which finished last weekend.

Alaphilippe was hotly tipped to retain his title in San Sebastian, and the race also looked set to feature a renewed battle between him and Ineos' Tour champion Bernal.

However, with the pair nowhere in the race, it was Toms Skujins and Evenepoel who made the burst away from the peloton as the race edged towards its climax, with Skujins darting forwards with 20 km left before being chased down by Evenepoel.

Evenepoel then broke away from Skujins as the final climb reached its summit, and he then held off the rest as he powered to a historic win.