The Racing Engineering driver, who should have won last year's feature race here but for a botched pitstop strategy, lapped in 1m57.996s in damp conditions with all drivers using the Pirelli wet tyres.
Robin Frijns was second fastest for Hilmer, but four tenths down on Leimer's best, on 1m58.430s.
Local hero James Calado was third fastest in his ART machine, juts a few hundredths shy of Frijns, with Julian Leal fourth in the second Racing Engineering entry.
Daniel de Jong was the surprise of the session, showing great pace in sector three in particular for MP Motorsport, and finishing the session in fifth, ahead of DAMS duo Stephane Richelmi and Marcus Ericsson.
Title contender Felipe Nasr was eighth fastest, 1.6s off the pace, but had been up near the top early on.
Not troubling the top of the times was championship leader Stefano Coletti, who was last of the 26 runners, 3.322s off the pace. Monaco winner Sam Bird ended the session 17th for Russian Time.
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