Sebastian Vettel gave the first real indication of the potency of this year's Red Bull Racing challenger by setting the pace on the opening day of the third Formula One group test of the year, at Jerez.

As intermittent rain once again disrupted proceedings in southern Spain, Vettel took the RB6-Renault to the top of the times with a lap of 1min 22.593secs as he racked up over 100 tours. Testing was heavily affected by the wet conditions, with heavy rain overnight and during the morning forcing initial runs to be carried out on wet tyres and curtailing the team's planned aero and mechanical set-up testing. A break in the weather over lunch provided some dry running in the early afternoon, before the return of the rain was accompanied by the brief loss of the official timing system.

"We got a lot of laps done and the car has been good and reliable," head of race engineering Ian Morgan revealed, "We've made a few small changes, but we didn't get through as much as the programme as we'd hoped because of the weather. Sebastian and the car were consistent on the long runs in the dry which is promising. We're continuing to make steady progress; the car is better than it was last week and every time we turn a wheel we seem to be moving forward a little bit."

The changeable conditions made progress frustratingly slow for everyone, as meaningful running could not be achieved without the arrival of either a rain shower or a red-flag period. However, McLaren made the best of the track time available to continue with the development of its MP4-25, making solid progress with the balance and set-up based largely on thorough evaluation of and response to the data gathered at last week's running at Jerez.

Although Lewis Hamilton caused an early red flag during the morning's running when his car was stopped by a minor systems error., he ran without problems thereafter and was able to achieve some encouraging lap times as he conducted several long runs, amassing 72 laps - and more than 300km - by the time the chequered flag fell, although he remained half a second off the pace.

Ferrari, meanwhile, followed up two days of on-track promotional work with Giancarlo Fisichella at the wheel by installing Felipe Massa in a brand new F10 chassis. The Brazilian headed out for a systems check shortly after 9am, while the track was still wet from overnight rain, but his programme had to be adjusted as the weather continued to work against the Scuderia for the remainder of the day. The new car didn't sport any major updates over last week's Jerez test, although minor aerodynamic and mechanical updates were in evidence ahead of a more comprehensive overhaul scheduled for next week's Barcelona test finale.

Massa completed a total 50 laps, the fastest of which put him third overall in a time of 1min 23.204secs. A small problem forced him to a halt on track at the end of the morning session, when he was sitting at the top of the times, but the day was otherwise uneventful as the team worked on evaluating tyres in the adverse conditions.

Sebastien Buemi again provided encouragement for Toro Rosso, posting the fifth fastest time in the course of 79 laps in chassis STR5-02. Looking on the bright side, the team made the most of the weather to acquire useful data regarding the optimum switching point between dry, intermediate and extreme wet tyres. Also on the agenda was a brake system evaluation and, fortunately, there were enough dry laps available for Buemi to complete the programme.

Pedro de la Rosa also used the day to try out all the different tyre specifications on Sauber's C29, completing 76 laps and winding up fifth fastest on another encouraging day for the reborn team.

"The conditions were obviously not ideal today but, overall, things went well for us, and we were able to complete all the work we had on the programme for today," the Spaniard reported, "However, we hope to get in some more laps in dry conditions because we still have a lot of work to do. The weather forecast is not very good for tomorrow, so we have to make the most of every dry window we get."

Test engineer Thomas Hunsicker confirmed that all tyre specifications - wet, intermediate and dry - were evaluated during the day, with de la Rosa working on optimising the set-up for the various tyres as well as for different fuel levels.

The Mercedes team began the third test with Michael Schumacher at the wheel of the MGP W01, but the programme was constantly adapted to suit the conditions. Schumacher began the day on wet tyres before switching to intermediates as a dry line began to emerge by late morning. He successfully completed a planned long run on dry tyres in the early afternoon before a further downpour soaked the track. Despite the interruptions, however, the seven-time champion completed 111 laps with a best of 1min 23.803secs that left him sixth fastest.

"The weather was really changing all the time today, which made it difficult to follow our planned programme and to have a clear picture of everyone's performance," Schumacher commented, "From the laps that we achieved, we have again confirmed the reliability of the car, and from the impressions and data gathered, we can say that the pace and performance looks very good. We have a strong basis to keep up the development this week here in Jerez and next week at the final test in Barcelona."

Force India bucked the trend of the day by running two drivers in its lone VJM03, with new test and reserve driver Paul di Resta getting his first taste of the 2010 challenger during the morning before Adrian Sutil took over at 4pm. A productive day working through set-up and control systems checks saw the Scot complete over 300km, while Sutil covered a additional 120km.

"Today was my first run in the new car and it felt pretty good," di Resta noted, "The objective was to get as much running as possible and we managed over 300km, which was a great effort, particularly given the bad weather conditions we had over the morning. It seemed to be drying out and then the rain would return again, so you were in a sort of no man's land with the tyres. Even with this type of weather though I could get an impression of the car. It seems stable and good all round, a genuine improvement on the car I drove last year. I'll be back in the car in Barcelona and am looking forward to it already."

Vitaly Petrov was at the wheel of the sole Renault, completing 55 laps as he took the chance to add further experience of piloting an F1 car in both wet and dry conditions.

"Another wet day in Jerez, with track conditions constantly changing, which made it difficult to learn very much," the Russian admitted, "We have to hope that tomorrow is better so that we can get a lot of laps on the car."

Chief engineer Alan Permane confirmed that it had indeed been a 'very frustrating day', especially for Petrov, who seemed to have suffered the brunt of the team's wet testing.

"Vitaly has had very little dry running so far, and the frequent showers meant that the track was wet for most of the day and we couldn't achieve a great deal of constructive work," he added.

Rubens Barrichello was among those to suffer gremlins as he continued Williams' preparation for the 2010 campaign, typically missing out on the best of the conditions and having to settle for tenth spot on the timesheet.

"Today, the track was generally damp, with only a few windows to run dry tyres," technical director Sam Michael confirmed, "Although we covered a good number of laps, we had a couple of small mechanical problems that cost us track time when the grip was at its highest."

Lotus Racing, meanwhile, made its debut at a group test following its launch in London last week, and enjoyed a successful session with test driver Fairuz Fauzy completing 76 laps without problems and completing the 300km needed to gain his FIA superlicence. The team was among the first cars on track in wet conditions and, despite a lack of power steering due to supplier issues, Fauzy successfully fulfilled his and the team's objectives for the day.

"It's been an excellent day for the whole team," chief technical officer Mike Gascoyne announced, "Firstly, I want to say a big thank you to Fairuz, who did an excellent job today with the first proper day of running. With so much going on, such as doing systems checks, it can be a pretty disjointed day for the driver and, when you add into that the changeable conditions, and also the fact we were running without power steering today, it could have been a tricky day for Fairuz, but he didn't put a foot wrong, so well done to him.

"With the car, we had no major reliability issues - a few niggles with overheating which were quickly solved and then the car ran pretty faultlessly. The power steering issue will be fixed for tomorrow so, all in all, a very successful first day and a busy evening ahead preparing the car for tomorrow."

The day proved extra encouragement for Lotus as Fauzy finished the day ahead of fellow newcomer Virgin Racing, with Timo Glock bringing up the rear of the field some six-tenths shy of the Malaysian.

With heavier rain forecast for Thursday, the teams will have to continue adapting their programmes as they attempt to put miles on the cars ahead of next month's Bahrain season-opener.

Vettel will complete his final day of testing at Jerez, before Mark Webber takes over on Friday, while Hamilton, Massa and Buemi continue for McLaren, Ferrari and Toro Rosso respectively. de la Rosa will be at the wheel again before Kamui Kobayashi takes over at Sauber for Friday and Saturday, while Nico Rosberg will take over the Mercedes from Schumacher. Petrov will conduct a second day for Renault, while Sutil and Barrichello remain in harness at Force India and Williams, After Fauzy's lengthy run on day one for Lotus, Heikki Kovalainen takes over driving duties for the next two days, before Jarno Trulli joins the action on Saturday.