Valtteri Bottas admits he expects the Mercedes-powered teams to be fighting together at the front of the field at the start of the season.

Mercedes, McLaren, Williams and Force India have all looked strong so far during the first six days of testing, with the new power unit running reliably. Having said he was impressed with the power available, Bottas admitted he felt confident after finishing half of his pre-season running.

When asked by Crash.net if Williams was likely to be facing Mercedes-powered opposition at the front of the grid this season, Bottas replied: "It could be.

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"I think Ferrari is also looking like a strong engine and reliability is also not too bad. We'll see how much Renault can improve; I think we should not underestimate anyone. But we are hoping that we will be in this top 10 group and my feeling is that at least at the beginning of the season it could be mostly Mercedes-powered cars, but let's see. I actually think it could be quite tight - the field - especially the top 15 or something."

Having completed over 100 laps on Thursday, Bottas said it was clear there was a good base to work from with the FW36 but feels there is much more to come from it.

"I think this car straight out of the box has been not bad. Balance-wise we have not been struggling with any big issues. Of course there are a lot of things to improve but when there are no problems and the car is working it's not bad to drive. For sure we can improve it a lot but it's nice to see that the long runs we did and the full race distance in the afternoon was not that bad. So far it's quite a good feeling."

With Felipe Massa's lack of running on Wednesday being attributed to a wiring loom issue, Bottas praised the way Williams has been responding to problems.

"It goes quickly, much quicker than yesterday! The day goes quickly and I'm very happy with the work the mechanics did solving the problems overnight. This was the first proper day for us without any problems; a clean day and we went through the program. I think we need to be happy with this and hopefully we can have these kind of days a lot more now."