Jenson Button admits McLaren need to find a way to get the most out of the hard Pirelli tyres if they want a shot at winning Sunday's Spanish GP.
Button experienced mixed fortunes on the heated tarmac of the Circuit de Catalunya on Friday.
On the one hand he was quick - quickest of all in fact - on the soft tyres, clocking a 1:23.399 to edge Sebastian Vettel by 0.164s.
But on the other, he struggled with the hard compound and was twice heard complaining of "terrible understeer" as he tried to get the most out of the Italian rubber.
His triumphs and struggled were highlighted in his own assessment as he referred as a "good" one but a "tough" one too.
"It was a good afternoon," the 32-year-old told Sky Sports News.
"But still I think a tough afternoon still trying to understand the tyres - and still don't quite understand them.
"The car seems to be working pretty well on the softer of the two tyres, but on the hard tyre I just can't get it working."
The 2009 World Champion further explained the problem, revealing that he is battling to get the tyres up to operating temperature.
He added: "I just can't get the temperature into the tyres.
"I don't know why that is but it's something to sort out for Sunday."