Jenson Button says pre-season testing has given little away about who will emerge quickest ahead of the 2015 Formula 1 World Championship season.

The Englishman is enduring a tough run up to the new year as McLaren-Honda battle persistent issues on the new MP4-30, but admits the time out of his car has been spent gauging the pace of his rivals at the Circuit de Catalunya.

However, while Button says it was clear that Mercedes, and to a lesser extent Williams, showed a marked advantage in pace last year, he says this year has been difficult to read and suggests it will be a close opening round in Melbourne.

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"I have had time to look at the screens but I haven't got a clue what is going on," he said. "In terms of who is quick and who isn't, it is really difficult to read. I think Mercedes will be quickest, looking at the long runs but Ferrari seems pretty strong, Williams is difficult to read, and Sauber looks good, so good for them - Red Bull too.

"It is tricky, you put the soft tyre on and some gain 1.5secs, others gain four tenths, so it is a bit random. I think it makes it great for the first race and exciting. You could read last year that Mercedes and Williams were quickest, but this year I haven't got a clue so it could be a mixed up field for the first race.

Regarding his own pre-season build up, Button is remaining optimistic about his chances for 2015, saying he expects the McLaren-Honda to make 'big steps forward' once it gets up to speed.

"We've had some pretty tough pre-seasons before and we have had it at McLaren," he added. "We had the hoover exhaust I think in 2010 that kept falling apart. Pre-season testing has a lot of the time been very tough but most of the time we can pull it together reasonably well.

"This year you will see, more than ever, the big steps forward, which is exciting, but it needs to start relatively soon."