A fit and well Lewis Hamilton topped the first practice timesheet in Hungary, the Mercedes driver producing his fastest lap early on in the session on the medium tyre.
Hamilton edged Red Bull's Max Verstappen by 0.165s and Ferrari's Sebastian Vettel who was just a hundredth of a second behind the Dutchman.
Valtteri Bottas's session was cut short at the outset by an engine misfire, forcing his Mercedes crew to change the Finn's unit for FP2.
A warm and sunny Hungaroring on Thursday provided a big relief to teams after last weekend's chaotic conditions at Hockenheim, but Friday dawned cooler and cloudy with rain fast approaching.
The first practice session kicked off with Robert Kubica and Nico Hulkenberg leading the field, but Hamilton laid down the first real marker amid a lot of activity as everyone sought to gain mileage under the menacing skies.
Light rain progressively set in but hardly discouraged the field from running, with Verstappen clocking in behind Hamilton and just in front of Sebastian Vettel and Charles Leclerc.
While Hamilton - on medium tyres - increased his lead with a 1m17.233s, teammate Valtteri Bottas was stranded in the Mercedes garage, his W10 the victim of an engine issue that would require a swap to a previously used unit, a move that sidelined the Finn for the remainder of FP1.
The rain, while not heavy, encouraged several drivers, among whom were Hamilton and Leclerc, to switch to intermediates. But the bulk of the field remained on slicks.
As the track returned to normal grip conditions, McLaren's Carlos Sainz jumped the two Ferraris to move up to fourth, while Hulkenberg snatched third, but Haas' Kevin Magnussen, on soft tyres, overhauled both to line up behind Hamilton and Verstappen.
The Dane's early pace offered an indication that his Hockenheim-spec car was faster than Grosjean's Melbourne configuration, as the Frenchman struggled to clock in among the top ten.
However, with just over 30 minutes to go, a productive effort from a soft-shod Vettel saw the German snapping at Hamilton's heels with a 1m17.399s that put him 0.166s behind the reigning world champion.
Behind, Pierre Gasly slotted back into fourth but almost half a second adrift from P3 Verstappen.
In the closing stages of the session, Leclerc improved slightly on the soft compound but remained sixth, well off the times of Hamilton and Vettel.
Hulkenberg, Lando Norris, Sainz and Alfa's Kimi Raikkonen rounded off the top ten when the checkered flag came out, but Sainz's session was cut short by a water pressure issue, a problem that could require another Renault power unit change in the McLaren camp.
As a reminder, the Spaniard is on the bubble regarding engine elements, so any change to a new component would equate to a grid penalty.
Outside of the top ten, Daniel Ricciardo was P11 follwoed by Grosjean and Toro Rosso's Dany Kvyat and Alex Albon.
Racing Point's Sergio Perze was 15th, edging Antonio Giovinazzi, Williams' George Russell, Lance Stroll and Kubica.
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