Ferrari's Sebastian Vettel set the fastest time, ahead of world champion Lewis Hamilton, as Formula 1 pre-season testing started in Barcelona.
Vettel's best lap was 0.47 seconds quicker than Hamilton, doing little to diminish the sense that Ferrari may offer a closer challenge in 2016.
Red Bull's Daniel Ricciardo was third fastest, a further 0.6secs behind, with Williams' Valtteri Bottas fourth.
Jenson Button managed sixth fastest for McLaren-Honda, 1.8secs off the pace.
Headline lap times in pre-season testing are notoriously unreliable indications of true competitiveness because the teams do not reveal the specifications the cars are running in.
Nevertheless, Vettel alternated fastest times with Mercedes' Hamilton throughout the day, giving the impression that Ferrari have, as promised, made a significant step forward this season.
Hamilton's team-mate Nico Rosberg, who drives the Mercedes on Tuesday, said: "Ferrari are looking strong already and definitely we are expecting them to be a big threat."
However, Mercedes were as imposing as ever. Hamilton managed more than 150 laps - twice as much as any other car - and was fast throughout the day.
The world champion, gunning for his third consecutive title and his fourth in all, said he was "very satisfied" with his first day in his new car.
"It is a great start to the first day of testing," Hamilton said. "To get it to perform as well as it has with the least amount of problems, the guys have done a great job.
"It feels very much the same as last year's. It is an evolution so it doesn't really feel any different, but that is a good thing. It feels good and we have had lots of laps so far."
Not such a happy place
The US-based Haas team, who are joining F1 this season, had a difficult debut, their new car suffering a front wing failure on the main straight while Romain Grosjean was travelling at around 200mph.
This sort of thing is not unusual - Force India suffered a series of structural failures at the Hungarian Grand Prix last year - but it underlines the steep learning curve Haas will be on this year, despite their close links with Ferrari.
However, Grosjean did say his first impressions of the car were "great" and expressed his optimism the failure would not badly disrupt the team's programme.
The Franco-Swiss did run again towards the end of the day, finishing with the eighth fastest time.
Meanwhile, McLaren's testing programme did not get off to a great start.
The team hope for a significant improvement after a difficult 2015 season with the new Honda engine, which was both uncompetitive and unreliable.
The car ran reliably, completing about 80 laps, second best among the new cars after Mercedes.
But Button's fastest time was set on the 'soft' tyre - which is significantly faster than the medium used by Vettel, Hamilton, Ricciardo and Bottas ahead of him.
Racing director Eric Boullier said the new car's performance on its debut was "not good enough" because of a couple of problems that restricted mileage.
Boullier said: "The first data we have gathered on the car are looking as though the numbers on the car is correlating, so that's good. So that means we know the car is better than last year."
He added that Honda was "clearly on the right path" and that it had made some "good improvements" to the engine.
Honda is running an initial version of its engine at this test and plans to introduce an upgraded design at the second and final test next week.
The Japanese company said they expect to make step by step progress throughout the year as they seek to close on Mercedes.
Britain's Jolyon Palmer, embarking on his maiden grand prix season, was slowest of all after a problem-hit day in the Renault.
What does it all mean?
Although this year will have the shortest pre-season testing programme ever, at just eight days, it is far too early to draw any firm conclusions about the season ahead.
First impressions are that predictions of a closer fight between Mercedes and Ferrari look like being correct - and that could mean the first genuine two-way title fight since 2012, when Vettel and Fernando Alonso, then at Ferrari, disputed the championship.
Even at this stage, it seems relatively safe to say that it would be a surprise if any other team regularly challenged the top two.
And it is clear McLaren - and particularly engine partner Honda - still have a significant amount of work to do to realise their ambitions of returning to winning ways.
1 Sebastian Vettel (Ger) Ferrari 1:24.939
2 Lewis Hamilton (GB) Mercedes 1:25.409
3 Daniel Ricciardo (Aus) Red Bull 1:26.044
4 Valtteri Bottas (Fin) Williams 1:26.091
5 Antonio Celis** (Spa) Force India 1:26.298
6 Jenson Button (GB) McLaren 1:26.735
7 Carlos Sainz (Spa) Toro Rosso 1:27.180
8 Marcus Ericsson (Swe) Sauber* 1:27.555
9 Pascal Wehrlein (Ger) Manor 1:28.282
10 Romain Grosjean (Fra) Haas 1:28.399
11 Jolyon Palmer (GB) Renault 1:31.006
*= 2015 car
** = development driver
Celis, Button and Wehrlein set best times on soft tyre; all others on medium