Ferrari's Fernando Alonso was the surprise pacesetter in first practice at the Singapore Grand Prix.
The Spaniard edged Mercedes drivers and title rivals Lewis Hamilton and Nico Rosberg, followed by the Red Bulls of Sebastian Vettel and Daniel Ricciardo.
The Ferrari, which can be faster than its true competitiveness on Fridays, was 0.122 seconds clear of Hamilton, who beat Rosberg by 0.027secs.
The Mercedes were just over 0.6secs quicker than Vettel.
Hamilton and Rosberg were closely matched throughout the session as they embarked on the latest chapter of their intense battle for the world championship at one of F1's most dramatic venues.
The Marina Bay track provides them with arguably the toughest challenge of the entire season, with a long, bumpy track and a race that runs almost to the maximum two-hour duration in hot and humid conditions under lights at night.
Friday's first session took place following a backtrack by governing body the FIA on a ban on teams giving advice to drivers over the radio.
The teams argued that preventing them giving guidance on operating the cars' complex systems would cause reliability problems and this will now be allowed until next season, although driver coaching is still forbidden.
The issue has been the focus of attention in the sport for the past few days, but Hamilton still made a mistake in asking for information on sector times while out on the track during the session. He was told: "We'll discuss that when you're back in the garage."
Red Bull have hopes of being able to challenge Mercedes this weekend, but on the basis of Friday's first session they may have to be more concerned with a battle with Ferrari for best of the rest.
Ferrari have struggled in the past two races, but Alonso said on Thursday he expected to be more competitive this weekend and that he was still confident they could wrestle back third place in the constructors' championship from Williams by the end of the season.
He will not be expected to remain fastest, but even allowing for the fact that Ferrari tend to use less fuel than their rivals on a Friday - equal to a lap-time advantage of about 0.5secs a lap - Alonso looked to be a threat for the Red Bulls.
Ricciardo had said that for Red Bull to challenge Mercedes they would have to qualify within 0.2secs of them, but they were some way off that in the first session.
There was more bad news for the team when Vettel pulled off with what he suspected was an engine failure right at the end of the session.
Team principal Christian Horner said the engine had suffered a sudden loss of oil pressure, but that Vettel would not have to take a 10-place grid penalty because it was an older engine.
Horner said an engine change took about three and a half hours, but the team would try to get it fixed in time for Vettel to do some laps in the second session, which starts at 14:30 BST.
Behind the Red Bulls, Toro Rosso's Jean-Eric Vergne - who is fighting for his future after discovering he is being dropped for next season - was sixth, ahead of Kimi Raikkonen's Ferrari.
The Finn was 1.727secs slower than team-mate Alonso and suffered a brake fire towards the end of the session.
McLaren's Jenson Button was eighth, ahead of the second Toro Rosso of Daniil Kvyat, Force India's Sergio Perez and McLaren's Kevin Magnussen, the only man with no experience of the track before the session.
There were no major incidents, but several drivers came close to accidents as they flirted with the forbidding walls around the Marina Bay circuit.