George Russell topped the times for Mercedes ahead of Red Bull's Max Verstappen and the two Ferraris in Thursday's opening practice at the Azerbaijan Grand Prix.
World championship leader Kimi Antonelli, in the second Mercedes, was fifth-quickest after pulling up during the session with hydraulics failure in hazy, warm conditions.
Russell clocked a best lap in one minute and 45.387 seconds to outpace four-time champion Verstappen by 0.400sec, with Charles Leclerc, chasing a fifth pole position in Baku and first win, adrift by a further 0.004sec.
Seven-time champion Lewis Hamilton was fourth with Oscar Piastri sixth for McLaren.
World champion Lando Norris endured a tricky session in the second McLaren with various car problems before winding up 14th, having run only 14 laps compared to 25 by both Ferraris. Antonelli only completed nine laps.
On his return to action with Red Bull, after a broken wrist, Isack Hadjar also had brake and engine problems on his way to running only eight laps to finish 20th.
Leclerc initially topped the times in 1:54.395, but they fell rapidly with Verstappen taking over as the pair traded best laps on hard tyres until after 20 minutes Antonelli took command in 1:46.265.
The Italian then came to a halt at Turn Seven, having lost hydraulics. As he evacuated, the marshals chose to squirt fire extinguisher into the car without clear reason as a Virtual Safety Car (VSC) was deployed for two minutes.
The track gradually 'rubbered in' to deliver more grip and incidents, with Verstappen locking up at Turn One before Russell avoided a crash at Turn Three with a spectacular reflex save into a run-off area.
Verstappen, seeking his third win on the fast streets of Baku, went top again before being leapfrogged by the Briton, thanks to a generous tow from two-time champion Fernando Alonso's Aston Martin.