SINGAPORE (AP) Red Bull youngster Daniil Kvyat recorded the fastest time in Friday's practice for the Singapore Grand Prix, and Ferrari's Kimi Raikkonen was second, indicating those teams are set to provide a challenge to previously dominant Mercedes this weekend.
Red Bull had not topped the timesheets for any Formula One session this season but the performance level was confirmed by Kvyat's teammate Daniel Ricciardo, who was third fastest.
Runaway championship leader Lewis Hamilton, who has taken pole position in 11 of 12 races this season, was in an unfamiliar fourth place - though only three tenths of a second off Kvyat's time - and his Mercedes teammate Nico Rosberg was seventh.
Conditions at the Marina Bay street circuit aid Mercedes' challengers, with Ferrari operating well in hot track conditions as evidenced by its wins in Malaysia and Hungary, and the nimble Red Bull thriving through the many low-speed corners.
The 21-year-old Kvyat has not qualified higher than fifth in his two seasons in F1 while Ricciardo has never taken pole position in his career despite boasting three grand prix wins, so a breakthrough could beckon in Saturday's qualifying.
Their strong practice performance came against the background of more off-track drama for the team, which is in the midst of a messy divorce from its engine supplier Renault.
With Mercedes refusing to step in as Red Bull's provider, it seems that Ferrari is the only supply option left, and Helmut Marko - trackside point-man of company owner Dietrich Mateschitz - reiterated on Friday that Red Bull could quit the sport if the Ferrari negotiations do not bear fruit.
''The decision is done,'' Marko said to Sky Sports. ''If we don't have a competitive engine we will leave Formula One.
''As long as you don't have a power unit which can compete, it doesn't make sense.''
The drivers, though, were less concerned with the off-track politics than with the on-track performance.
''We expect the silver cars (Mercedes) and the red ones (Ferrari) to turn it up tomorrow, but hopefully we can keep the pace we showed to get up as far the grid as possible,'' Ricciardo said.
Mercedes team principal Toto Wolff was not as confident of an improved performance in Saturday's qualifying and was genuinely concerned about the disappearance of his team's season-long speed advantage.
''We are not happy with the performance today,'' Wolff said. ''We have not made the tire work the way we should on the one-lap performance, and on the long runs we have seen some spectacular performances by Red Bull.
''We need to get our heads together and assess what's happening.''
Raikkonen outperformed his Ferrari teammate Sebastian Vettel - who was fifth fastest - and continued the strong one-lap performance that saw him qualifying second in the previous race in Italy.
McLaren's Fernando Alonso also put in an eye-catching performance, clocking the eighth-best time, less than a second off the top of the charts. That was a marked improvement for the team, which has had a frustrating year as new engine supplier Honda plays catch-up in its return to the sport.
American driver Alexander Rossi, who will make his F1 race debut on Sunday for the Manor team, provided the major incident of the two 90-minute sessions, colliding heavily with a barrier at the left-hand turn under a grandstand, and snapping off both right-side wheels.
Rossi climbed out of the car unaided and appeared unhurt. The subsequent red flag ended the first session three minutes short.
The crash required a gearbox change, which meant Rossi missed most of the second session, as did his teammate Will Stevens, who also collided with a barrier.
