Nico Rosberg admitted his Mercedes team was surprised by its big advantage over its rivals following Friday practice for the Italian Grand Prix.
The German had been over 1.5 seconds slower in the morning.
"Yes, we are surprised," said Rosberg, who finished just 0.021 seconds behind Hamilton in the afternoon.
"We look really quick here at the moment, I am pleased with that.
"Lewis has been quick up until now but it got very close at the end there as we were trying the qualifying runs. That is okay, so we are going in the right direction.
"It has not gone my way up until now. But of course I will try and tomorrow is another shot and it is very close today and for sure there is a good opportunity tomorrow."
Hamilton added: "It has been a good day. A testing day. Lots of things to try. The car has been feeling good so far this weekend and it's great pace for the team so need to keep it together."
Although Mercedes has introduced an updated engine after having used all its remaining development tokens, Hamilton said it felt the same as before.
"It just feels the same. It is more reliability changes to improve reliability," he said.
"The engine feels fantastic because it is a fantastic engine. It still feels the same."