Sergio Perez, McLaren's new signing, set the fastest lap of the day by edging world champion Sebastian Vettel during Formula One testing at the Circuit de Catalunya.
The Mexican driver crossed the line 0.349 seconds quicker than the German's Red Bull on day two of the second round of pre-season testing.
Lotus' Kimi Raikkonen lost out on morning practice due to gearbox problems but came back to set the third fastest time.
Lewis Hamilton, whom Perez replaced at McLaren, was fourth for his new team Mercedes, while Fernando Alonso, who says he has been working on physical fitness this pre-season, steered his Ferrari to fifth.
The 11 F1 teams are in Catalonia testing cars on the new Pirelli tires that all must use this season.
Running the new MP4-28 car, Perez completed 97 laps ahead of teammate Jenson Button who is due to test on Thursday. The team said the aim of the morning was for engineers to get "a better understanding" of the car's "reaction to a series of mechanical set-up changes."
The Mexican found the best grip and was quickest round the track using Pirelli's soft compound, the team said.
Vettel had set the fastest time in the morning but was forced to cut short testing after lunch when he stopped on the track because of a loose wheel after exiting from a pit stop. Race stewards stopped practice while his RB9 was brought back to the Austrian team's boxes.
"We'd done several pit stops and obviously at the last one the wheel wasn't fixed properly," Vettel said. "Obviously we wanted to run a bit longer, and that didn't work," the triple world champion said after completing 84 laps.
Ferrari said on its Facebook page that Alonso also had to limit practice to 76 laps because of a problem with an exhaust pipe.
Lotus said Raikkonen had been frustrated but relaxed in what it said was a "mixed day" of testing.
"Yes, it's frustrating and no driver wants to hang around a race track with nothing to do," the 2007 champion said. "We had a gearbox issue and it took the crew a long time to fix it. Sometimes these things take a long time and that's pretty normal when you have a test."
Hamilton, who suffered a high-speed crash on Feb. 6 when his back brakes failed while testing at Spain's southern Jerez track, hit no reliability problems as he completed 121 laps, the most of any driver on Wednesday.
Hamilton, whose time was 0.029 seconds slower than Raikkonen's, found the best grip on Pirelli's hard tire, while Alonso was quickest using the medium compound.
Testing at the north-eastern track ends on Friday.
Who is your favourite V8 Supercars driver?Related story: (See story)