Max Verstappen was fastest in the only practice session for the 2023 Azerbaijan Grand Prix from Charles Leclerc as Alpine suffered a poor start to their weekend.
Most drivers switched to the soft tyres for the final part of the session, when Perez set the fastest first and second sectors to originally go 0.391 seconds quicker than Verstappen.
But, the times continued to tumble as the track evolved and Verstappen took top spot as the chequered flag came out by 0.037 seconds.
Sergio Perez held the fastest time for most of the session, but ended 0.139 seconds behind his teammate, with Carlos Sainz 0.584 off the pace.
McLaren have brought the car concept they wanted at the start of the season to Baku, after an underwhelming opening three events, and the new floor seems to have paid off with Lando Norris in fifth.
Nyck de Vries was a surprise sixth for AlphaTauri, in front of Aston Martin pair Lance Stroll and Fernando Alonso.
Alpine and Mercedes the big losers
The opening half an hour saw plenty of incidents, most notably a hydraulics failure for Pierre Gasly, which led to a fire and a red flag with 40 minutes left of the session.
Gasly tried to get back to the pit lane but was told to stop in the middle sector and Alpine decided to not run Esteban Ocon for the remainder of FP1.
Lewis Hamilton and George Russell were 11th and 17th after a brake problem limited their track time.
Hamilton set his first flying lap 32 minutes into the session and both drivers opted to not run the soft tyre, so it’s difficult to judge their performance in Baku.
Yuki Tsunoda limped back to the garage after nudging the wall at Turn 3 and appeared to be going too fast on a flat tyre on his in-lap.
Qualifying for the main Grand Prix takes place at 14:00 UK time as the new sprint format is used for the first time this season.
Saturday sees another qualifying session (09:30 UK) for the sprint race, with the sprint itself (14:30 UK) rewarding championship points for the top eight finishers.