Red Bull could face the biggest challenge yet to their unprecedented unbeaten sequence of F1 race wins at the Singapore GP after a difficult start to their weekend - which Max Verstappen admits was "way worse than we expected".
Verstappen and team-mate Sergio Perez finished outside the top six on the timesheet in the night-time practice session at Marina Bay, unusually trailing the pacesetting Ferraris by a gap of seven tenths a second.
Struggling to get the RB19 car to their liking on a twisty circuit where braking stability is all important, Verstappen said after finishing eighth fastest: "Just struggling a lot with the balance of the car.
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"We tried quite a lot of things in FP2; some worked, some didn't. Never really got the car together. So quite a few things to figure out tonight, basically."
Verstappen, on a record run of 10 consecutive wins and fast closing on his third world title, added: "I will try to of course improve it, it's quite a big gap.
"Ferrari is very fast; I think we're just way worse than we expected."
While Red Bull have certainly not always finished Friday at the top of the timesheet this year - and few are likely to believe they will not be back in the mix for pole position when qualifying comes around at 2pm live on Sky Sports F1 on Saturday - it's clear there are a few more challenges so far here than usual, particularly on lower fuel.
Getting qualifying right is also imperative on a street circuit such as this, where the race has only been won four times in 13 years from outside the front row.
"I think there are some interesting bits going on that we need to figure out overnight," said Perez, who at one stage of the second session said he feared he was going to crash such was his trouble with braking.
"We seem to be struggling quite a bit with the rear end of the car, especially in FP2. So plenty of things to look at and hopefully we can come up with the best possible set-up because we know that qualifying is very important."
Perez, a place ahead of Verstappen on the timesheet in seventh, added: "We did expect the Ferrari to be very strong around here.
"We are just too far away so hopefully tomorrow [Saturday] we are able to close the gap a bit more But I do expect quite a challenge."
Sainz 'doesn't believe' Red Bull are in trouble | Leclerc 'surprised' by Ferrari pace
Ferrari were one-two in both practice sessions on Friday and were seven tenths quicker than Red Bull in the night session, which will represent the same conditions for qualifying and the race.
Carlos Sainz topped that timesheet in that session and revealed he "tested a couple of things" in practice that paid off.
"You always need to arrive at Singapore very open-minded. We have seen in the past very weird performance swings at this track. It's a very particular track," said Sainz.
"So far it looks like it's suiting our car well and it's going well with the way we hit the ground running in FP1 and just fine tuned the setup a bit for FP2 and be a bit quicker. It looks good but I believe the track is going to change a lot tomorrow [Saturday] - it's going to ramp up the grip a lot and it's all about who will find the perfect setup [on Saturday]."
Asked if Red Bull are in trouble, the Spaniard added: "No I don't believe it. When you look at their long run pace they have the strongest car. Once they sort out their one-lap pace they will be there fighting for pole. Hopefully we can be in that fight.
"It also looks like Aston Martin, McLaren and Mercedes should also be in that fight, so we are in for an exciting day tomorrow and hopefully we can get the best out of it."
Charles Leclerc has taken the last two pole positions in Singapore and has also finished second in both of those races.
Leclerc says the car felt good "straight away" and his side of the garage learned from their mistakes at Monza in the build-up to qualifying.
"We are surprised [about our pace] but we are not getting carried away either. It's only Friday and we expect the other teams to show more of their true potential tomorrow [Saturday]," said Leclerc.
"We can't get carried away but it's true that we did not expect this kind of performance, on a Friday at least. It's a good start but now we need to focus on ourselves and produce, hopefully, the same [on Saturday]."
"It's going to be very close, for sure. I expect the others to be much faster tomorrow [Saturday], so let's see. I hope we won't have any bad surprises and be less competitive than what we think we are at the moment. But for now everything looks good."
Mercedes seek to build on 'best Friday of year' | Alonso: Red Bull will be fast when it counts
Instead of Red Bull it was Mercedes who got closest to Ferrari on Friday with George Russell 0.235s off Sainz's benchmark and team-mate Lewis Hamilton a further two tenths back in fifth.
Both drivers felt Mercedes had enjoyed their best Friday of the year so far and hoped overnight tweaks could put them in a position to battle for pole position.
"It's been a really good day. It's been a smooth sailing day. We started on the right foot, the car progressed from P1 to P2 which is not too often the case. I feel we have a good platform to work off," Hamilton said.
"We're a little bit off compared to the Ferraris but we'll work overnight to see if we could close that gap. I think the top three or four teams are all very, very close, within a tenth or so of each other [on race pace].
"I'm definitely excited for [Saturday], we just got to make sure we're sensible with the changes we make."
Russell added: "It's definitely pretty close between five teams at the moment which is pretty exciting so we'll have to work hard overnight and gain those last milliseconds ahead of [Saturday].
"Ferrari are looking favourites at the moment. I don't think they were maximum power unit as well so they've probably got a couple more tenths in the pocket. The fight is on probably for the second row."
Splitting the two Mercedes was Fernando Alonso's Aston Martin and the Spaniard is predicting an extremely tight qualifying session on Saturday and that Red Bull will come on strong when it matters.
"Today [Friday] is just getting confident with the car and the track. I think Ferrari is probably out of reach again like Monza, a little bit too fast. Red Bull will be fast tomorrow [Saturday] when it counts," the two-time world champion said.
"It's very tight - you can be P5/P6 or you can be out of Q3 and P12 because within two tenths there are a lot of cars. It is going to be tight and we have to be ready for anything and nail the lap in Q2 and Q3."
Lando Norris also managed to get his upgraded McLaren ahead of the two Red Bulls in Practice Two and said of his Saturday prospects: "[We're in the fight for] good positions.
"I think we're way too far from the Ferraris. The Ferraris are way too fast for us at the minute.
"Coming in we knew we were probably second, third or fourth quickest, in that bunch. I was expecting Ferrari to be quickest by quite a margin which they are so it kind of makes sense. We're not far behind, I think if we put in a mega lap we can get close - we're in the fight with Mercedes, Aston and Red Bull so it looks like it will be an exciting qualifying."
So just how competitive could the Singapore pole fight be?
Sky Sports F1's Karun Chandhok
"Of course, across the season Red Bull have had the best package, and they've got the best chance of being on the front row.
"But I think Ferrari are in it, Mercedes are in it, McLaren particularly with Lando (Norris), and Fernando Alonso [for Aston Martin].
"So, I think we have four teams who could all end up within three or four tenths - then it's up to the drivers. They've got to deliver the lap. Get the tyre in the right window for the right lap and then deliver it."
Sky Sports F1's live Singapore GP schedule
Saturday September 16
10.15am: Singapore GP Practice Three (session starts at 10.30am)
1pm: Singapore GP Qualifying build-up
2pm: Singapore GP Qualifying
3.45pm: Ted's Qualifying Notebook
Sunday September 17
11.30pm: Grand Prix Sunday: Singapore GP build-up
1pm: THE SINGAPORE GRAND PRIX
3pm: Chequered Flag: Singapore GP reaction
4pm: Ted's Notebook
Next up for the F1 circus are the stunning streets of Singapore under the lights. All sessions from the Singapore GP will be live on Sky Sports F1 this weekend. Stream F1 on Sky Sports with NOW. Cancel anytime