Ferrari's Felipe Massa set the fastest time so far in pre-season testing on the third day of running at Jerez.
The Brazilian was nearly 0.887 seconds ahead of Nico Rosberg's Mercedes, with Red Bull's Sebastian Vettel third.
Rosberg completed more than 140 laps, an encouraging improvement for Mercedes after problems restricted the car to just 29 laps over the first two days of the test.
Thursday was the 2013 F1 debut for Vettel and Lotus's Kimi Raikkonen.
The three-time world champion took over the Red Bull from team-mate Mark Webber, who drove on Tuesday and Wednesday, and Vettel ended the day 1.173secs off Massa's pace-setting lap of one minute 17.879.
Red Bull team principal
“All the cars you see are evolutions of 2012 cars. I think you're going to have the usual suspects hammering it out again this year”
Raikkonen, third in the championship last year behind Vettel and Ferrari's Fernando Alonso, was fourth fastest ahead of Toro Rosso's Jean-Eric Vergne.
Force India's British simulator driver James Rossiter, 29, took over from race driver Paul di Resta for the afternoon and set the sixth fastest time, ahead of Jenson Button's McLaren.
The day's activities in southern Spain ended five minutes early when Button's car stopped out on the track as McLaren ran it dry - a standard procedure for teams to evaluate how well their fuel pick-up system is working.
Mexican novice Esteban Gutierrez, having his first run in the Sauber, was eighth fastest ahead of Williams new boy Valtteri Bottas, who was driving an modified 2012 car.
Force India had planned to put Di Resta back into the car for the final part of the day but an exhaust failure forced them to end running early.
Mercedes were trying to make up for time lost to the reliability problems in the first two days, when Rosberg suffered an electrical fault and Hamilton a brake failure.
Rosberg's laps total surpassed the number done by any car last year, which was his then-team-mate Michael Schumacher with 132.
But that was done in a 2011-spec car so for Rosberg to manage so many in one day in a new car is an encouraging sign for the team. The team were also trying a new front-wing design, which Rosberg said was "progress".
Horner wants to extend 'halcyon period'
Rosberg said: "I have no idea how fast we are, but if look at what Ferrari have done I'm not worried about their time. We made up a lot of what we lost in the last two days and I'm sure will be the same tomorrow.
"We've just been unfortunate the last two days. They were problems that could easily have been avoided. Just unlucky and today showed we had a very reliable car."
Massa, who hands the car over to development driver Pedro de la Rosa for the final day on Friday, said: "It's always nice to see your name in first position but it doesn't count for anything.
"I'm sure there are a lot of very quick cars on the track and I'm not sure if we are in that position.
"We need to work a lot to be ready for the first race and make the car quicker all the time. I don't know if we are ready now but it was a positive start."
Vettel said he was encouraged by his first day in an F1 car this year.
"The target was to do many laps," he said. "So far we can be pretty happy.
Mercedes recovered well on Thursday after the problems of the first two days. Nico Rosberg did a lot of laps and they will have made progress analysing set-ups on the car and the effectiveness of the new front wing design he used.
Rosberg complained of understeer - a lack of front-end grip - but all the cars had that to one degree or another. That's because Jerez is very hard on the tyres and they were suffering from 'graining', where the rubber tears and then the loose bits stick to the surface, reducing grip.
The yardstick for Mercedes will be when Lewis Hamilton gets in the car on Friday. Rosberg is used to how the Mercedes feels after three years with the team, but Hamilton is used to the McLaren. His feedback will be crucial to the team understanding where they are.
Most people came away fairly happy, but although Felipe Massa set the fastest time of the week so far many are still viewing Jenson Button's slower lap on Tuesday as the benchmark.
It came on the 'hard' tyre, whereas Massa was on the 'soft'. Allied with the abrasive surface, the softer tyre can give grip that the car doesn't have and teams can get an artificial reading of how fast the car is.
"Mark had a good two days, I took over today. It was about cleaning off the rust. The car is working well.
"It's difficult to judge the speed, but the reliability was pretty good and I feel pretty happy with the car.
"I expect it to be at least as tight as last year. The regulations haven't really changed and if you are in the front it is hard to find something else that makes you quicker."
Lap times in winter testing are a notoriously unreliable means of predicting true form as it is impossible to know the specifications in which the cars are running.
Red Bull team boss Christian Horner predicted another competitive season.
"We've had reasonably stable regulations over the last four years and very stable regulations into this year," he said.
"All the cars you see are evolutions of 2012 cars. We had some great close racing last year and I think you're going to have the usual suspects hammering it out again this year."
FIRST PRE-SEASON TEST, DAY THREE, JEREZ, SPAIN, FASTEST TIMES
1. Felipe Massa (Brz) Ferrari 1:17.879
2. Nico Rosberg (Ger) Mercedes 1:18.766
3. Sebastian Vetttel (Ger) Red Bull-Renault 1.19.052
4. Kimi Raikkonen (Fin) Lotus-Renault 1:19.200
5. Jean-Eric Vergne (Fra) Toro Rosso-Ferrari 1:19.247
6. James Rossiter (GB) Force India-Mercedes 1.19.303
7. Jenson Button (GB) McLaren-Mercedes 1:19.603
8. Esteban Gutierrez (Mex) Sauber-Ferrari 1:19.934
9. Max Chilton (GB) Marussia-Cosworth 1:21.454
10. Valtteri Bottas* (Fin) Williams-Renault 1:21.5785
10. Charles Pic (Fra) Caterham-Renault 1:22.352
11. Paul di Resta (GB) Force India-Mercedes 1:23.729
* = 2012 car