Tom Pidcock said he was "surprised" to beat Wout Van Aert on the home straight of the Flanders Classic, the Brabantse Pijl, on Wednesday for his first win at Ineos since turning professional in February.
The 21-year-old Briton outlasted the field on an energy-sapping course where Italy's Matteo Trentin came third.
Van Aert, 26, and Pidcock had the lead on the field with the younger man lurking at the Jumbo-Visma rider's wheel over the final 200m.
"I waited and waited and when Wout went, I kind of got a little boxed in and then rushed and came past him," Pidcock said.
"Beating Wout van Aert in a sprint is a surprise, but luckily for me he went early," he added.
In his first season at Ineos, Pidcock has impressed with huge rookie performances at the Strade Bianche (5th) and the Kuurne-Brussels-Kuurne (3rd).
Pidcock, Van Aert and Trentin dropped the other contenders on a narrow, cobbled climb 37km from home before their cat-and-mouse run in.
UAE Team Emirates rider Trentin described Pidcock as a worthy winner after the Italian briefly led alone before allowing the pair to catch him to distance the rest of the peloton.
"When I saw Van Aert and Pidcock coming across I decided to ease up a little bit and try and recover," Trentin said.
"The legs were a bit tired in the end and the strongest guy won.
"I look forward to coming back racing on these roads at the World Championships later in the year."
Dutch rider Gijs Leemreize of Jumbo-Visma became the first man to be disqualified for using a dangerous riding position.
The downhill supertuck frame sitting position and the time-trial arms on the handlebars stance are now both banned by the International Cycling Union.