The prolific Slovak, who claimed the biggest one-day victory of his career in Gent-Wevelgem last month, followed it up by taking his eighth victory of the year in the Ardennes semi-classic.
The 23-year-old, who before the race apologised to the podium girl whose bottom he pinched after finishing second in Ronde Van Vlaanderen with a bouquet of flowers, narrowly edged out Belgian Gilbert, winner of Brabantse Pijl in 2011.
The BMC rider launched the head-to-head sprint but Cannondale's Sagan pipped him to the line.
The 200km race featured 25 short climbs with a parcours similar to Sunday's opening Ardennes World Tour race, the Amstel Gold, for which Sagan will start as favourite.
Following the start in Leuven, an escape group formed just before the quarter mark of the race and featured Kenny DeHaes (Lotto Belisol), Nikolas Maes (Omega Pharma – Quick-Step), Ben Hermans (RadioShack Leopard), Youcef Reguigui (MTN Qhubeka), Jan Ghyselinck (Cofidis) and Ireland's Philip Deignan (UnitedHealthcare).
They built up a 4:30 lead but that advantage was down to 75 seconds with 55km remaining, by which time Reguigui had been dropped.
Three 23.4 kilometre laps, with five climbs on each lap, completed the race and moves began to be made in the peloton with Radioshack's Stijn Devolder, Bjorn Leukemans (Vacansoleil), Paul Voss (NetApp-Endura) and David Malacarne (Europcar) bridging the gap to the lead group with only Maes and Dehaes remaining of the original escapees.
The race completely split after a massive turn of pace from Gilbert's team-mate Greg van Avermaet inside the final 20km.
Sylvain Chavanel, a former winner of the race, along with Simon Geschke (Argos-Shimano), Gilbert, Sagan and Van Avermaet joined the leaders with 12km to go and an 11-man group held off chasers including last year’s winner Thomas Voeckler and Milan-San Remo champion Gerald Ciolek on the run-in to the finish in Overijse.
Maes and Van Avermaet went off the front with 2km to go but they were caught by Sagan, on the final climb of the Schavei, inside the final kilometre.
The Slovak then accelerated and only Gilbert was able to hold on to his wheel. The Belgian almost immediately opened out the sprint, only to find Sagan able to react and then come off his wheel to take the victory.
Leukemans was three seconds back in third after almost reaching the front two at the summit of the final climb.
Chavanel, Geschke. Van Avermaet, Malacarne, Devolder, Voss and Dehaes completed the top 10.
- Sports & Recreation
- Nikolas Maes