Nico Rosberg headed Lewis Hamilton by nearly half a second in first practice at the Malaysian Grand Prix.
A lock-up by Hamilton into Turn Nine on his second lap was the difference between the two after they set almost identical times on their first laps.
That prevented Hamilton improving on his time and forced him into the pits.
The session was interrupted by a bizarre fire for Renault's Kevin Magnussen, which continually reignited in the pits as fuel leaked.
In the end, Hamilton set his best time on his second run on the medium tyre - which is as much as a second slower than the soft.
The Mercedes were a second clear of the Ferraris, with McLaren's Fernando Alonso fifth ahead of the Red Bulls of Daniel Ricciardo and Max Verstappen.
In a disjointed session, there were few indicators of form between the two title rivals at Mercedes, as Rosberg heads into the weekend leading Hamilton by eight points in the championship with six races remaining.
For most of the session, they were lapping within tiny fractions of a second of each other, and their positions in the timing lists was defined by their very first runs on a track that has been entirely resurfaced and reprofiled at many points for this year's race.
Their first laps were like that - Rosberg setting a one minute 35.793 second lap and Hamilton on 1:35.798.
But after Hamilton locked up at Turn Nine and ruined that set of tyres, Rosberg continued and lowered his time to a 1:35.227.
Kimi Raikkonen was the fastest Ferrari driver, 1.088secs slower than Rosberg and 0.016secs ahead of team-mate Sebastian Vettel, who berated his team on his first run for failing to balance his tyres correctly.
"The vibrations are so bad I can't really see at the end of the straights," he said. "I don't know how you can balance a set and do it so badly."
A giant leap for McLaren?
Alonso's time was set on the soft tyre, just as those of Rosberg, Kimi Raikkonen and Sebastian Vettel ahead of him, and was encouraging for a team running an upgraded version of the Honda engine.
The Spaniard will start from the back of the grid as a result of using the new power-unit, with Honda due to decide after the first session whether to use it in the second session, which starts at 07:00 BST.
Team-mate Jenson Button was 10th fastest, on the medium tyre.
The session was red-flagged after just a few minutes when Magnussen's car caught fire in the pits.
Renault said there had been a leak from a fuel breather pipe, and that Magnussen had been running some experimental parts that were not on the car of team-mate Jolyon Palmer.
Magnussen did not get out again as the car was cleaned up and repaired, his team-mate Jolyon Palmer ending up 19th fastest, ahead only of the Manors of Esteban Ocon and Pascal Wehrlein.