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Jenson Button says the McLaren-Honda Formula 1 team's top-10 pace in Friday practice for the Mexican Grand Prix was unexpected.

The 2009 world champion is running Honda's latest specification engine for the first time this weekend.

It was fitted as part of a scheduled double change and as a result, he will receive a 50-place grid penalty for Sunday's race.

The Briton completed very little running in first practice after suffering a high voltage sensor failure and then spent the opening 45 minutes of the second session in the garage because of a deployment issue.

But he eventually hit the track, completing 25 laps and setting the ninth fastest time, 1.578 seconds adrift of pacesetter Nico Rosberg, and one place behind team-mate Fernando Alonso.

"It wasn't bad," said Button. "I think it was a little bit unexpected.

"We have [tactical] engine penalties here because we thought it would be a really tough race for us - and it still will be. But slightly more positive.

"It's a little bit like Sochi and the low grip of Austin as well, and we seem to work OK in those conditions."

While Button was pleased with how the 'phase four' power unit performed, he conceded there was still plenty more work to do to maximise the package.

"It's louder," he said. "In terms of power, I don't know, we're still a long way off.

"We're 16km/h off the quick cars so still a lot of work to do.

"But it's progress and at the moment it's still running reliably, which is good."

Alonso will also take a grid penalty, after McLaren fitted his 11th internal combustion unit of the season, and therefore believes qualifying pace will be irrelevant.

"In qualifying, unfortunately whatever position we get we will start last with the penalties," he said.

"So we will concentrate tomorrow on the race pace and tyre management.

"We saw a lot of degradation today, more than expected, so we need to put some attention on it.

"We keep making progress, the car keeps improving with the new parts, and now we need to see what we can do in the race."