Mercedes team boss Ross Brawn was not fully satisfied with the performance of his two cars during practice at the Japanese Grand Prix.
Both Nico Rosberg and Lewis Hamilton suffered flat spots on their tyres during the afternoon, which Brawn admitted led to the team having to adapt.
He told reporters: "We had a slightly messy session this afternoon which didn't quite run as smoothly as we might have hoped.
"Our pace looked competitive this morning, a little less so this afternoon, but as always on Friday the varied fuel loads up and down the pit lane complicate things somewhat."
Hamilton also voiced concerns about the amount of tyre wear his Mercedes may produce across the race weekend.
