Carlos Sainz led Charles Leclerc to a Ferrari one-two in Friday practice at the Singapore Grand Prix with Max Verstappen down in eighth place.
Sainz was just 0.018 seconds up on his team-mate, who was on course to go top but lost time in the final sector.
Mercedes driver George Russell was third, 0.235secs off the pace, ahead of Aston Martin’s Fernando Alonso.
Verstappen and team-mate Sergio Perez, who was seventh fastest, were both complaining of a lack of rear grip.
They were behind Lewis Hamilton’s Mercedes and Lando Norris’ McLaren, which has a major aerodynamic upgrade this weekend.
Even on the race-simulation runs later in the session, the Red Bulls did not come into their own.
They were the fourth fastest car, with Alonso setting the pace, from Mercedes, then Ferrari and then Red Bull.
Runaway championship leader Verstappen said before the track action started that he expected Singapore to be one of the tracks on which Red Bull would struggle more than normal and so far that is proving an accurate assessment.
Verstappen had a rear snap at Turn 13, the exit of the Anderson Bridge, while Perez complained over the radio that he felt like he was going to crash on every corner entry.
Team principal Christian Horner said between the Friday sessions: “This track is going to be one of the toughest of the season for us. The characteristics don’t play naturally to the strengths of the car and we are going to have to play right at the top of our game to keep this winning run going.
“High speed has been strong for us and combined [braking and turning corner] entries, whereas 90-degree corners have been more of a challenge for us. But we can still be there.”
Verstappen is on a record-breaking run of 10 consecutive victories and Red Bull have won every race this year.
