Lotus's Romain Grosjean set the fastest time for the second day running at the final pre-season test in Barcelona.
Going out late in the day on soft tyres, he displaced Toro Rosso's Jean-Eric Vergne by 0.514 seconds.
Vergne was 0.235 seconds clear of world champion Sebastian Vettel's Red Bull, with Ferrari's Fernando Alonso fourth.
Lewis Hamilton was ninth fastest for McLaren, some way off the pace after spending much time in the garage as the team worked on the car.
McLaren have a raft of new parts to try out at this test as they run the car close to the specification in which it will appear at the first race on 18 March in Melbourne, Australia.
Hamilton said he was looking forward to trying the new parts when he gets back in the car on Sunday. Jenson Button takes over on Saturday.
"We need the pace and the predicted time that the new parts are going to bring to compete with the other guys at the front," he said.
"Of course the other guys will make a step forward, too; it's whether we can make a bigger one."
Red Bull continued with their low-key but quietly impressive pre-season programme, while Alonso completed a race-distance simulation for the first time this year.
The Spaniard was out at much the same time as Grosjean, who was also doing a race simulation.
The two appeared relatively closely matched, although the Ferrari clearly had the edge on outright pace and Grosjean spent the majority of his time on soft tyres while Alonso generally used the harder and slightly slower mediums.
Alonso, whose fastest time was less than 0.1 seconds slower than Vettel's, was also trying out a new front wing. All the top three teams are assessing new parts aimed at significantly improving the performance of their cars.
Vettel admitted that Red Bull had their own significant upgrades to try over the weekend.
"It's normal in the last couple of days of testing [that] you always bring some bits on the car, so the car is as close as what we race in the first race," he said.
"Other people bring some new parts and so do we, but even then it is hard to judge whether you are quick because people play around with fuel loads and the time of day."
Grosjean's feats over the last two days are a perfect example of how headline times in pre-season testing are a notoriously poor indication of true form.
It cast many minds back to last year's pre-season, when the team - then known as Renault - also looked quick.
They started the 2011 season strongly but their form quickly tailed off and they finished the year fifth in the constructors' championship. Their aim this year is to go one place better.
Caterham showed a decent turn of speed for the first time this winter, despite Kovalainen still recovering from a violent bout of food poisoning on Thursday, which he said had caused him to lose 3kg in one day.
Force India, who have looked the most impressive midfield runner so far in testing, suffered a setback when Nico Hulkenberg had to abort a planned race simulation test after the car suffered a gearbox problem on its first lap.
The German was the cause of one of eight red flags for drivers stopping out on the track.
Two were triggered within 20 minutes of each other in the morning session by Mercedes driver Michael Schumacher - one because the car stopped and the second because the veteran slid off the track.
Final pre-season test, Barcelona - day two fastest times
1. Romain Grosjean (Swi) Lotus-Renault - 1 minute 22.614
2. Jean-Eric Vergne (Fra) Toro Rosso-Ferrari - 1:23.126
3. Sebastian Vettel (Ger) Red Bull-Renault - 1:23.361
4. Fernando Alonso (Spa) Ferrari - 1:23.447
5. Heikki Kovalainen (Fin) Caterham-Renault - 1:23.828
6. Kamui Kobayashi (Jpn) Sauber - 1:23.836
7. Nico Hulkenberg (Ger) Force India-Mercedes - 1:23.893
8. Michael Schumacher (Ger) Mercedes - 1:23.978
9. Lewis Hamilton (GB) McLaren-Mercedes - 1:24.111
10. Bruno Senna* (Brz) Williams-Renault - 1:24.925
11. Pastor Maldonado* (Ven) Williams-Renault - 1:25.801
*Bruno Senna and Pastor Maldonado shared testing duties