Lewis Hamilton and Sebastian Vettel neck-and-neck in Jerez
Red Bull's Sebastian Vettel and McLaren's Lewis Hamilton set closely matched times on the third day of the first pre-season test at Jerez.
Lotus driver Romain Grosjean set the pace for a 2012 car, with Vettel second just 0.167 seconds ahead of Hamilton.
The fastest time of the day was posted by Nico Rosberg's 2011 Mercedes.
Fernando Alonso had his first taste of the new Ferrari but spent the day on aerodynamic measurement work.
The Spaniard's fastest time was 1.145secs adrift of Vettel's.
Grosjean, who is entering his first full season of Formula 1, took the Lotus over from team-mate Kimi Raikkonen, who drove on the first two days of the test. He set the fastest time of the test so far in one minute 18.419 seconds early in the day.
Grosjean's best time was 0.806secs slower than Rosberg's - a 1:17.613.
Testing times are a notoriously unreliable guide to real form, but that applies even more so to Rosberg's times as the rule changes applied in F1 over the winter to slow the cars down mean his times have no real relevance to those of the 2012 cars.
Grosjean set his best time while running in a configuration that would boost performance - on new tyres and what is believed to have been relatively light on fuel.
But BBC F1 technical analyst Gary Anderson said it was nevertheless an "impressive" performance.
"Sure, he did it early on and it's obviously low-ish fuel," Anderson said, "but it shows a confidence in the car as he didn't mess about, he just got in and did it."
Vettel's fastest time of 1:19.297 was 0.113secs slower than team-mate Mark Webber managed on Wednesday at the end of two days' running.
Hamilton was not up to speed as quickly as Vettel, but closed in on the Red Bull's pace towards the end of the morning.
Hamilton said: "My initial thoughts were pretty good. It was quite a tough day where you had to go through a lot of test items, but it went pretty smoothly.
"No nasty surprises. Nice surprise is that it feels reasonably quick, so that's a good thing, although obviously it can always be better.
"The baseline we have is something we can really work with. I know the guys back in the factory will take a lot from these four test days and translate that into something new for the next test."
Hamilton and Vettel both said they could feel the lack of grip as a result of the ban on blowing exhaust gases along the rear floor of the car to increase downforce.
Vettel said: "It's been two months since I drove and it takes a couple of laps to get back in the rhythm.
"The car feels fine, it's more or less what we expected, quite a big loss in grip because of the change of regulations but you get used to it fairly quickly and then you start to explore how the new car feels.
"I think we can be quite happy so far. We had a couple of issues that stopped us for a while. It's normal with a new car - you don't take unnecessary risks. The more you run, the more you understand."
He added: "It's clear it should be very tight again, if not tighter than last year."
Ferrari emphasised that they were not looking for headline lap times as they concentrated on "a few runs for aerodynamic measurements. This is a very important job, aimed to get more understanding of car's behaviour."
Alonso's team-mate Felipe Massa had said after finishing his test on Wednesday that the car still needed "a lot of work".
Ferrari's work was delayed by a hydraulic leak that kept Alonso in the pits for more than an hour.
Nevertheless, Alonso's best of 1:20.412 was 0.042secs faster than the Brazilian had managed before.
He was separated from Hamilton by the Toro Rosso of novice Jean-Eric Vergne and Sergio Perez's Sauber.
Force India were not able to do any testing after reserve driver Jules Bianchi crashed the car on his first lap.
The team described the damage as "minor" but did not have sufficient spare parts to repair the car and are flying some out from the UK so they can resume running on the the final day of the test on Friday.
FASTEST TIMES, DAY THREE TESTING, JEREZ
1 Nico Rosberg (Ger) Mercedes 1:17.613*
2 Romain Grosjean (Swi) Lotus 1:18.419
3 Sebastian Vettel (Ger) Red Bull-Renault 1:19.297
4 Lewis Hamilton (GB) McLaren-Mercedes 1:19.464
4 Jean-Eric Vergne (Fra) Toro Rosso-Ferrari 1:19.734
5 Sergio Perez (Mex) Sauber-Ferrari 1:19.770
7 Fernando Alonso (Spa) Ferrari 1:20.412
8 Bruno Senna (Brz) Williams-Renault 1:21.293
9 Guido van der Garde (Bel) Caterham-Renault 1:23.324
* = 2011 car