Jenson Button admits he wasn't expecting McLaren-Honda to end the first day of free practice for the Mexican Grand Prix with both cars inside the top ten.

Having predicted a troubled weekend given the Honda-powered machine's top speed disadvantage down the 1.2km home straight, the tighter, slippery infield is proving better suited to the MP4-30 strengths, allowing both Fernando Alonso and Button to break into the upper echelons in FP2.

Even so, it wasn't an easy day for Button after an engine change in the morning meant he only hit the track during the second-half of FP2, which means he will also take a grid penalty come Sunday. Despite this, he was encouraged by the car's form as a whole.

"A little bit unexpected," he admitted. "We have engine penalties because we thought it would be a really tough race for us and it will be still a tough race with deployment and things.

"Slightly more positive then, it's a little bit like Sochi, a little bit like the low grip of Austin and we seem to work OK in those conditions."

Getting use of the new specification Honda power unit this time after Alonso ran it in Austin, though he wasn't enamoured by its speed figures, he says he is pleased it is running reliably.

"In terms of power, I don't know, we're still a long way off. We're 16-18kph off the good cars. There's still a lot of work but it's progress and at the moment it's running reliably, which is good."