Lewis Hamilton claimed Ferrari were bluffing after Sebastian Vettel blitzed the competition in posting the fastest time by a distance in pre-season testing in Barcelona on Thursday.
The German four-time world champion clocked 1min 19.024 around a sun-kissed Circuit de Catalunya despite appearing to ease off in the third sector of his fastest lap.
Hamilton was three tenths slower on the same ultrasoft tyre compound used by Vettel.
"I think Ferrari are bluffing and that they are a lot quicker than they are showing," said the Englishman, who had a busy 24 hours in the Catalan capital.
Hamilton was putting the Mercedes W08 to the test at 9am local time (0800GMT) after being pictured celebrating Barcelona's sensational Champions League comeback against Paris Saint-Germain with Barca star Neymar into the early hours in the Catalan capital.
Mercedes have dominated for the past three seasons with Hamilton and retired world champion Nico Rosberg winning all but two of 21 races last year.
However, despite his skepticism, Hamilton does expect to be pushed all the way by Vettel as Ferrari have closed the gap following an overhaul to the sport's regulations.
"It's difficult right now to say exactly who is quicker," he added. "They are very close if not faster.
"Whatever the case, I can't wait to go out and compete. Just a couple more weeks to go."
Vettel also showed the reliability of the SF70H by logging a marathon 155 laps to also record the most miles for the day.
But Vettel insisted Mercedes remain the firm favourites for the drivers' and constructors' titles.
"If you look at how many laps Mercedes have done; look at historically how slow they go in testing, how much they were able to ramp it up for the races then it's clear," said Vettel.
"They are very fast with their long run pace. They're the ones to beat."
McLaren's miserable preparation for the season, which gets underway in Melbourne on March 26, continued as electrical issues saw Belgian rookie Stoffel Vandoorne stop twice out on the track.
Vandoorne completed just 48 laps as once again McLaren and their problematic Honda engine were left at the back of the paddock in terms of mileage completed.
However, McLaren executive director Zak Brown rejected rumours that the British team could look to break ties with Honda.
"We have a long-term contract," he told Sky Sports.
"We have won a lot of championships with them, they have the right people, we have the right people, so we are going to work through it together."