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Lewis Hamilton has given a cautiously optimistic assessment of his new Mercedes after his first proper run in the car in pre-season testing.

Hamilton, who has joined Mercedes from McLaren, said: "We have some work to do but it doesn't feel like a disaster."

Mercedes ended last season struggling even to score points, while McLaren had Formula 1's fastest car.

"It seems we're not in a bad starting place so it must be a big improvement for [the team]," Hamilton said.

"I feel like it's a good platform, a good foundation to work on."

After a brief outing on Wednesday was curtailed by brake failure, Hamilton said he felt the car was lacking aerodynamic downforce, a point he made again on Friday.

"Definitely. [I'm] just going to keep on asking for downforce and hope the guys continue to do a great job back at the factory," he said.

Hamilton and team-mate Nico Rosberg were restricted to less than 30 laps between them over the first days because of reliability problems, but the German managed 148 laps on Thursday and the car ran reliably again for Hamilton on Friday as he completed 145 laps.

"It's been good to get some laps in," Hamilton said. "Obviously yesterday Nico got 148 laps in. I think that's the most I've ever seen. Today to get some running in, it's great to get a feeling for the car."

Team boss Ross Brawn said: "We had some problems the first two days which were disappointing, but since then it's been running well. We've been learning a lot about the car and about working with Lewis.

"It's still very early in terms of understanding what he wants from a car perspective, and of course we're at one circuit for one day.

"He's clearly a person who enjoys being involved at all levels and that's a great motivation for the team.

"He's been working hard with the engineers this test. He's been here on the days when Nico's been driving with a headset and listening to what's been going on, so [he has been] everything I hoped he would be."

Brawn said optimism in the team was "reasonably strong".

He added: "There is an awful lot to do but both drivers are putting a lot of energy into the team.

"We know what happened last year; we know what we need to do this year. We need to quietly get on with it, not panic and make sure we do things in the right way and we have two great drivers to help us do that."

The fastest driver on the day was Lotus's Kimi Raikkonen with a lap of one minute 18.148 seconds, which was 0.027secs quicker than Force India's Jules Bianchi.

"The car feels good," said the 2007 world champion. "We are pretty happy with how the handling is and how things are running.

"For sure there are things we have to improve but that is normal. It is only the first test.

"For us it is more important to have a good feeling and know what we have done and learn from those [than to set the fastest time].

"It's early days but it's a good car and I think we have a good baseline like last year to work on."

World champion Sebastian Vettel's Red Bull was third. Hamilton was sixth overall, 0.757secs behind Raikkonen.

Vettel said: "I'm quite pleased with the car but about the performance we can't really say anything because we don't know what fuel the others were running. At this stage it's all about ourselves and trying to understand the car."

Raikkonen's time was 0.269secs slower than the fastest time of the week, set by Ferrari's Felipe Massa on Thursday.

Ferrari's new development driver Pedro de la Rosa had his first run in the car on Friday, although he lost the morning's running while the team repaired a gearbox failure.

Testing was suspended for 50 minutes in the early afternoon after a pot hole developed at Spain's Jerez track between Turns 10 and 11, where drivers run wide over the kerb.

It was repaired with quick-drying concrete and the end of test was extended until 1630 GMT.

FIRST PRE-SEASON TEST, DAY FOUR, JEREZ, SPAIN, FASTEST TIMES

1. Kimi Raikkonen (Fin) Lotus-Renault 1:18.148

2. Jules Bianchi (Fra) Force India-Mercedes 1:18.175

3. Sebastian Vettel (Ger) Red Bull-Renault 1:18.565

4. Esteban Gutierrez (Mex) Sauber-Ferrari 1:18.669

5. Jean-Eric Vergne (Fra) Toro Rosso-Ferrari 1:18.760

6. Lewis Hamilton (GB) Mercedes 1:18.905

7. Sergio Perez (Mex) McLaren-Mercedes 1:18.944

8. Valtteri Bottas (Fin) Williams-Renault 1:19.851

8. Pedro de la Rosa (Spa) Ferrari 1:20.316

10 Charles Pic (Fra) Caterham-Renault 1:21.105

11 Luiz Razia (Brz) Marussia-Cosworth 1:21.226

12. Paul di Resta (GB) Force India-Mercedes 1:23.435

* = 2012 car