Max Verstappen and Lewis Hamilton were third and fifth fastest in pre-season testing as the row over Mercedes' car took a new twist.
Red Bull team principal Christian Horner said the Mercedes design complied with the rules a day after being quoted saying it was "illegal".
Horner said it was an "extreme" rules interpretation, but: "It looks like it ticks all the [legality] boxes."
He insisted his earlier quoted comments about the car "weren't made".
The row has overshadowed the first two days of testing in Bahrain, which ended with Verstappen 0.13 seconds quicker than Hamilton.
The world champion trailed pacesetter Carlos Sainz of Ferrari by 0.479secs, with Lance Stroll's Aston Martin third ahead of Hamilton, as a number of senior figures emphasised that it was not possible to accurately judge form from testing.
Haas driver Kevin Magnussen set a time 0.3secs faster than Sainz in the extra hour his team was allotted at the end of the day to make up for time they lost on Thursday as a result of a freight delay.
There is one day of testing left on Saturday before the new season starts at the same Bahrain track next weekend.
What's the row with Mercedes about?
Mercedes turned heads at the start of the Bahrain test on Thursday when they unveiled an aerodynamic upgrade to the car that first ran at the opening test in Barcelona two weeks ago.
The upgrade features minimised sidepods and turns the mandatory side-impact structures into aerodynamic wing sections, with the wing mirrors mounted on them.
Germany's Auto Motor Und Sport quoted Horner expressing doubts about the legality of the new design, which Mercedes have introduced at this week's final pre-season test in Bahrain.
In the article, Horner says: "Those are not mirror mounts, but two wings. In addition, they have built vertical baffles on top. They have nothing to do with the mirror mount.
"From our point of view, Mercedes has gone a step too far. That does not correspond to the spirit of the regulations. For us, these wings are illegal."
On Thursday, Red Bull at first said the quotes were "incorrect" before clarifying that they were not "official".
The journalist in question, the veteran reporter Michael Schmidt, stands by the story.
Attending the official news conference on Friday, Horner was asked about the apparent contradiction in his position.
He replied: "Comments have been quoted that certainly weren't made. The car is innovative and it's an interesting solution. The Mercedes car looks like it complies with the regulations. It is just a different interpretation, a different solution."
He added: "There's not really anything that defines the spirit of the regulations; it either complies or it doesn't. That's not really for us to judge.
"[Governing body] The FIA have access to all of the drawings. A design like that would have surely been submitted in advance. It's an interesting concept, it's a radical concept. So, is it quick or not? Only time will tell, but in terms of its compliance, that's very much an FIA matter."
Who is quick?
Horner said it was "impossible to tell" which teams were in the best competitive shape.
"What you can say is that it's only going to be next Saturday [in qualifying at the Bahrain Grand Prix] you'll get the first snapshot as the fuel comes out," he said, "[when] people run as close to the weight limit as they can.
"I don't think Mercedes have shown their hand yet. Ferrari look the form team. Hopefully we can be in there as well. It looks like McLaren have a strong car. Alpine have a strong car."
Hamilton said it was a "tough" day for Mercedes, and that the team were still struggling with the aerodynamic "bouncing" problem they and many other teams experienced at the first test.
He added that it looked like Ferrari were in good shape "and maybe Red Bull".
Sainz rejected claims from Mercedes driver George Russell that Ferrari looked in the best shape so far.
"It is typical Mercedes, typical George - hype up the others and then come to the first race and blow the competition away," the Spaniard said.
"If it was the first season they did it, I would maybe believe them, but they have done it for five, six years now and they keep surprising us in the first race. So as you can imagine I don't believe much."
Sainz implied that analysis of the GPS speed traces available to all the teams suggested that Mercedes were absolutely competitive.
"Already in GPS we can also see what they are doing," he said, "and, yeah… I am not going to say much."
McLaren admitted that they were being hamstrung by a front brake problem that required new parts before it could be fixed.
Their driver Daniel Ricciardo missed a second day of running because of illness, forcing Lando Norris to drive again.
It was later confirmed that the Australian has tested positive for Covid-19 and will miss the final day with Norris stepping in once again.
McLaren said in a statement that Ricciardo was "continuing to isolate in accordance with local regulations".
The team added: "Under these regulations, Daniel will be released in time for next weekend's Bahrain Grand Prix. Daniel is already beginning to feel better."
Ricciardo will have to return a negative test before he is allowed to compete in the season-opening race.
Williams missed most of the day's running after a fire in the rear brakes caused Nicholas Latifi to spin off during the morning session and there was a failure at the rear as the fire was extinguished. The team did not manage to get out again.
Fastest times, day one, Bahrain test
1 Kevin Magnussen (Den) one minute 33.207 seconds
2 Carlos Sainz (Spa) 1:33.532 seconds *
3 Max Verstappen (Ned) Red Bull 1:34.011*
4 Lance Stroll (Can) Aston Martin 1:34.064 *
5 Lewis Hamilton (GB) Mercedes 1:34.141 *
6 Esteban Ocon (Fra) Alpine 1:34.276 *
7 Charles Leclerc (Mon) Ferrari 1:34.366
8 Lando Norris (GB) McLaren 1:34.609
9 Sebastian Vettel (Ger) Aston Martin 1:36.020
10 Yuki Tsunoda (Jap) Alpha Tauri 1:36.802
11 Valtteri Bottas (Fin) Alfa Romeo 1:36.987 **
12 Mick Schumacher (Ger) Haas 1:37.846 **
13 George Russell (GB) Mercedes 1:38.585 **
14 Nicholas Latifi (Can) Williams 1:39.845 **
15 Zhou Guanyu (Chn) Alfa Romeo 1:41.805 **
* = C4 tyre; ** = C2 tyre. All other times on C3. C5 is softest, C1 hardest.
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