By Alan Baldwin
BARCELONA (Reuters) - Lewis Hamilton set the pace for champions Mercedes on the opening day of Formula One's first pre-season test on Monday with his new car gobbling up the laps with a hunger that others could not match.
Hamilton, already favorite for a fourth title this year following the surprise retirement of reigning champion team mate Nico Rosberg, completed 73 trouble-free laps of the Circuit de Catalunya in the afternoon sunshine.
New Finnish team mate Valtteri Bottas did 79 laps in the same car before lunch -- the equivalent of more than a race distance each.
Only Ferrari came close, with Sebastian Vettel chalking up a whopping 128 laps on his own.
Unlike Mercedes, who won 19 of 21 races last year, Ferrari did not share their driving duties. Finland's 2007 champion Kimi Raikkonen will test on Tuesday.
While times were of secondary importance, Hamilton's best lap of one minute 21.765 seconds was a second quicker than the fastest in testing at the Barcelona circuit in 2016 and 0.113 better than Vettel.
"It's been a positive day for the team. Lots of laps and lots of information that we've gained today," said Hamilton.
"There's so much more downforce, which is something we always wanted," he added.
"This is day one so we've not done any work to get the balance perfect. Today is just really about ticking off all the lists, all the checkpoints and getting mileage for reliability."
While Mercedes collected a rushing torrent of data for the engineers, it was more of a trickle for Red Bull and McLaren who hit immediate setbacks.
McLaren's Fernando Alonso failed to set a timed lap in the morning, due to an oil systems problem, and ended the day with only 29 laps done.
"It's definitely not a perfect start of the winter testing," the Spaniard, whose Honda-powered team have struggled for the last two seasons, told reporters.
"We are disappointed, we are sad to arrive at the first day and not be able to run. We are aware of the time we lost today. We have four days for each driver before the world championship and now one day is gone.
"You've been working for three months on your physical fitness, on the simulator, on the sponsor commitments and launching the car... and now you hit the track and do an installation lap and something breaks down."
Over at Red Bull, Australian Daniel Ricciardo completed four laps in the morning before his RB13 was sidelined by a sensor problem, bringing out the first red flag of the year.
The car was returned to the pits shrouded in a tarpaulin on the back of a flatbed truck. Ricciardo went out again in the afternoon and did 50 laps in total.
Veteran Brazilian Felipe Massa was third fastest for Williams, with 103 laps under his belt.
The new cars, wider and with bigger front and rear tyres, are expected to be several seconds a lap quicker than last year's once the racing starts in Australia on March 26.
(Editing by Alison Williams and Clare Fallon)