Daniel Ricciardo was in the top three in both free practice sessions on Friday ahead of the Malaysian Grand Prix.
The Australian, who won the race last year, was second fastest in the damp first session, but team-mate Max Verstappen set the pace.
And in the dryer second session, Ricciardo was third fastest but eight tenths of a second behind the pace-setting Ferraris of Sebastian Vettel and Kimi Raikkonen, with Verstappen in fourth, two hundredths of a second slower.
The second session though was cut short when a drain cover was dislodged and Haas driver Romain Grosjean drove over it, shredding his rear tyre and sending him into the barriers. On safety measures, the practice session was red flagged with around 15 minutes remaining.
Crucially, with the championship battle between Vettel and points leader Lewis Hamilton hotting up, both Red Bulls were ahead of the Mercedes driver in both sessions, with the Brit appearing to struggle at the Sepang circuit.