Red Bull were tipped by George Russell and Charles Leclerc as the team to beat after the first day at the Bahrain Grand Prix in Malaysia.
Red Bull drivers Isack Hadjar and Max Verstappen finished the day second and fourth fastest but those positions appeared to belie their underlying pace.
Ferrari's Leclerc was fastest by 0.099 seconds from Hadjar, while Verstappen was 0.257secs off the pace and behind McLaren's Lando Norris.
Lewis Hamilton was fifth fastest and 0.305secs off the pace in the second Ferrari.
But Verstappen did not do a qualifying simulation run on the soft tyre, and Leclerc said Hadjar had not optimised his.
Leclerc said: "Everybody is very close. It's been a very difficult day overall.
"The track conditions were very tricky. The track was quite dirty and very slippery so things will evolve and change a lot from today until tomorrow and we have to be on the good side of that anticipating that in the best possible way.
"I felt quite good with the car but I still feel like the Red Bull is standing out this weekend so far.
"Isack did some mistakes in the last part of the lap and Max didn't do the lap on the soft tyres.
"I don't think we are the most competitive and also the Mercedes is a bit difficult to understand. I'm sure once Mercedes put everything together they will be very strong. Normally when they put something on the car it works."
Verstappen, who chose not to do a soft-tyre run because rain was forecast and he wanted to prioritise race preparation, said his day had been a "decent start" and "positive".
Technical director Pierre Wache added: "We were not very optimistic starting the weekend. The main aspect was to evaluate the package we put in Baku in different type of corners than low speed.
"It looks to be working better than the previous package. Relative performance is difficult to judge but we were surprisingly happy with where we were today."
Mercedes drivers George Russell and Kimi Antonelli were only seventh and eighth fastest in terms of headline times as they evaluated a major upgrade on their car, the team's first since the Canadian Grand Prix in June.
Russell said: "I'm feeling the downforce in some of the corners but with this amount of tyre overheating and degradation that trumps everything. So we just need to get on top pf that overnight,. That's more of a set-up rather than the upgrade itself and go from there.'
Asked whether he could compete for pole position on Saturday, Russell said: "Currently, Verstappen looks very fast and the race pace of the McLaren looked very quick but never say never."
The Briton trails Antonelli by 66 points in the championship after his victory in Azerbaijan last weekend.
On the race simulation runs, Verstappen also looked like the car to beat, taking stint length and tyre wear into account.
But the top four teams all looked to be in close company after a day evaluating tyre wear in the tropical temperatures and on an abnormally abrasive track, where F1 cars last raced in 2017.
Racing Bulls' Liam Lawson and Alpine's Pierre Gasly were ninth and 10th.
Three drivers have grid penalties. Hadjar will drop five places from his qualifying position for exceeding the permitted number of engine parts.
And Arvid Lindblad of Racing Bulls and Alpine's Franco Colapinto will be at the back, the Briton for engine penalties, and the Argentine because of a 10-place engine penalty on top of the five-place penalty he was given for causing a crash at a restart in last weekend's Azerbaijan Grand Prix.
F1 has returned to this track, regarded as one of the best layouts among the new generation of circuits that came on stream at the end of the last century and into this one, because of the circumstances around this year's calendar.
The Bahrain and Saudi Arabia races were cancelled at the beginning of the year as a result of the war in the Middle East.
But Bahrain were keen to hold their race and Crown Prince Salman bin Hamad bin Isa Al Khalifa negotiated to hold the race in Malaysia.
A bunch of Bahraini-related activities are being held at the track, while the trophy presented will be the Bahrain Grand Prix trophy and rosewater rather than champagne will be sprayed on the podium.
