Fernando Alonso topped the times for Ferrari in Friday morning's opening rain-hit free practice session ahead of this weekend's Belgian Grand Prix.
The two-time world champion clocked a fastest lap in one minute and 55.198 seconds in variable cool and damp conditions with only short spells when the drivers attempted to run on slick tyres.
The Spaniard's lap lifted him just .026 seconds clear of nearest rival Paul Di Resta of Scotland and his Force India team-mate Adrian Sutil of Germany who was third fastest.
But on a day of capricious conditions at the majestic circuit in the Belgian Ardennes, the outcome was hardly a signal of what may lie ahead as the teams return to work following the month-long mid-season European summer break.
As Ferrari responded to the challenge - or at least Alonso, as under-pressure team-mate Brazilian Felipe Massa was only 11th fastest - McLaren also rose to the forefront with both Englishman Jenson Button and Mexican Sergio Perez impressing in the difficult weather.
Button wound up down in 14th place, one ahead of former team-mate and compatriot Lewis Hamilton of Mercedes, but Perez was fourth fastest ahead of German Nico Rosberg in the second Mercedes and defending triple world champion Sebastian Vettel of Germany in the Red Bull who was sixth.
Vettel's retirement-bound team-mate Australian Mark Webber finished up 19th after a frustrating morning that produced few pointers to the next sessions or the race.