It is the first time in his F1 career the diminutive Briton has not been powered by a Ferrari engine.
"I went to the (Mercedes) factory and I was impressed," Massa is quoted by Italy's La Repubblica.
There are rumours Mercedes has come out so strong in 2014 because the German carmaker has invested four times more money than Ferrari.
"I don't know whether it is four times," Massa said, "but definitely to see them at work made an impression on me."
However, he denied that he deliberately shopped for a Mercedes-powered team for 2014.
"No, but with a regulation change that is about the engines, you know that they (Mercedes) know what they are doing.
"And from the moment I arrived at Williams I felt very wanted, which is a fantastic feeling."
Massa's last comment suggests that he no longer felt loved at Ferrari.
"I will not speak badly of Ferrari," he insisted. "I was there many years and I lived some beautiful moments. And some very bad ones as well."
He does not hide that Hockenheim 2010, when he was told 'Fernando Alonso is faster than you', was the low point.
"They did not let me win a race that I deserved," said Massa. "It was not just the team order that hurt me, but the fact that I had come back from a very bad accident. It would have been very important to me."
Asked if he often behaved 'too loyally' to Ferrari, he admitted: "Yes, maybe I did. But now it's the past and it doesn't matter.
"I remember the wonderful years at Ferrari, and the friendships with many people."
Massa, 32, said he thinks Mercedes has the best 2014 car, and when asked 'Hamilton or Rosberg', he answered: "Nico Rosberg."
But when asked who he would put money on for the title, Massa grinned: "I don't like talking about myself..."