Felipe Massa was impressed with Ferrari and Mercedes after the opening day of testing as they were "straight away competitive" in Spain.
Lewis Hamilton set the pace on Day One of pre-season testing as the Mercedes driver posted a 1:21.765 to edge Sebastian Vettel by 0.113s.
The German, though, set his fastest time in the SF70H using Pirelli's medium tyres.
Massa was third in the new Williams FW40, 0.311s down on Hamilton's pace-setting time.
"I think they are quite strong – Ferrari and Mercedes," said the Brazilian.
"It's just the first day, but what I can say is that [both teams] were straight away competitive, straight away putting some good lap times.
"And when a car is competitive straight away, it means something for the championship…"
He added: "Ferrari did a very good lap time on the medium tyres. Their car looks nice.
"Mercedes also looks very nice. Red Bull had a problem today… many cars had problems today."
As for the new regulations, the 2008 championship runner-up said he enjoyed his first taste of the new rules as the cars now allow drivers to "attack a lot more."
He told the official F1 website: "It's a big difference [compared to 2016]. It was a nice feeling – definitely from a driving point of view.
"The car has a lot more downforce, has a lot more grip on the corners, the tyres have more grip and the mechanical grip is a lot higher, so the behaviour is quite different in the corners – you can attack a lot more, bring more speed to the corners."
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