Sebastian Vettel led the way on the penultimate day of F1 testing at the Jerez de la Frontera circuit in the extreme south of Spain.

The German, who took his maiden win at the Italian GP last weekend with Scuderia Toro Rosso, was again behind the wheel of a Red Bull-Renault RB4 - like he was on Wednesday - and despite being delayed by an unscheduled engine change, he finished proceedings at the top of the times, on a 1 minute 18.001 seconds.

Vettel spent the morning working through an aero and a mechanical comparison test, which apparently produced 'positive results'.

In the end though he only did 37 laps in total, as rain later on in the day, saw the team end its programme prematurely.

Nick Heidfeld ended the session 'best of the rest', just under two tenths slower on a 1m 18.167s. The German worked on the mechanical and aerodynamic set-up of the BMW Sauber F1.08 early on, evaluating different configurations for the remaining races of the 2008 season.

When rain arrived around lunch time, he continued running, albeit with his focus altered: "As we might have some rain during the forthcoming races we took the opportunity to work on the car's wet weather set-up," he explained.

Lucas di Grassi came in third for Renault, the only other runner to break the 1 minute 19 second barrier. He ran mostly in the morning, as the team continued with its aero programme and evaluated various updates for the R28.

"It was quite cool today but the track conditions were good in the morning and there was definitely better grip than yesterday," said di Grassi. "I'm also getting used to the car and the more laps I do, the more confident I feel. We didn't do any running in the wet, but we were able to test a few more parts this morning that the team will be taking to the race next week."

"A slightly challenging day for us as we didn't get all the test results we wanted to and the day finished early due to poor weather," added Renault's chief test engineer, Christian Silk. "However, we managed to cover the main items this morning, so it's not been too bad and we've got some interesting data to go through this evening. Hopefully it will be dry tomorrow and we can get a good final day of testing in."

Further down the order McLaren's Pedro de la Rosa and Honda's Alex Wurz rounded out the top five, split by less than three tenths of a second.

de la Rosa concentrated on the aerodynamic proving of components to take to the final four races. He also continued with the KERS programme that was started during yesterday's session.

With rain falling during the final two and half hours of the day though, McLaren elected to remain in the garage and to focus on preparations for the final day of running tomorrow.

Of the rest Takuma Sato was sixth for Toro Rosso, his first day back behind the wheel of an F1 car following the demise of Super Aguri F1. He worked on set-up, while also doing an aero test.

"At the start of the day we were collecting aero data," said Sato. "Then by the end of the morning we began timed runs, so I could really feel the car and I started to enjoy the laps.

"Unfortunately in the afternoon it started to rain so we had to stop, that was a great shame, as I was looking forward to proper running in the afternoon. Although I couldn't complete the day due to the weather conditions, I really enjoyed driving the STR3. I would like to say a big thank you to Red Bull and to the team for giving me this great opportunity."

The only other runners on track were Toyota's Kamui Kobayashi and Williams' Kazuki Nakajima.

After yesterday's work in preparation for Singapore, Kobayashi switched his focus to the other races remaining on the 2008 calendar, working on set-up and systems. He was interrupted by a hydraulics issue in the morning before the rain took hold.

"After yesterday's trouble-free day of work, today was more difficult as the rain showers took running time away from all the teams," he noted. "In addition we had a hydraulics issue in the morning so we missed quite a lot of track time overall. Still, we made good progress while we were out on the circuit and this has been a solid test for me."

"We planned to spend today working on set-up and systems for the last few races but it was a disrupted day," continued Toyota test team manager, Gerd Pfeiffer. "We lost 90 minutes of running in the morning with the hydraulics issue. Then at 1pm we had the surprising sight of a shower here, which interrupted proceedings further. Kamui was just going out for another run when the rain returned, washing out the rest of the day."

Testing now concludes on Friday, with Nick Heidfeld, Pedro de la Rosa, Alex Wurz and Lucas di Grassi all due to stay on, while RBR will use Sebastian Buemi, Toyota will bring in Timo Glock, Williams will use Nico Rosberg and Sebastian Vettel will revert back to the Toro Rosso fold.