McLaren's Jenson Button set the pace in a rain-hit practice for the Monaco Grand Prix on Thursday but kept a wary eye on rivals Lotus.
Button, a winner around the treacherous metal-ringed street circuit in his 2009 title season with Brawn GP, skimmed the barriers and roared past the luxury yachts bobbing in the harbour with a fastest time of one minute 15.746 seconds on super-soft tyres.
"We have not done a lot of running for a Thursday at Monaco so hopefully Saturday will be a bit better," he said looking forward to qualifying, with Friday a rest day in the principality.
"The cars that looked very fast for me were the Lotuses. Their runs whenever they are out look very competitive, so I'd say the Lotus is the car to beat so far this weekend."
Button's time was set in the dry as drivers got in quick laps in the first 15 minutes before the rainclouds darkened the skies and made conditions slippery.
Frenchman Romain Grosjean burnished his hopes of becoming his country's first Formula One winner in Monaco since Olivier Panis in a wet 1996 with the second-fastest time for Lotus in both sessions.
"I like Monaco, it's good fun. The car is going well at the moment so let's see what we can do later on. It's important to have a car that you are confident with and work every single detail because it makes such a big difference," he said.
A win by Grosjean or team mate Kimi Raikkonen would give Formula One a first of having six different winners from six different teams in the first six races.
Ferrari's Fernando Alonso had showed his Monaco mastery in the morning with a pace-setting 1:16.265 in a sunny session cut short in a cloud of smoke when Heikki Kovalainen's Caterham blew its engine in the tunnel.
The Spaniard, twice a winner in Monaco with Renault and McLaren, was fourth in the afternoon without fitting the super-soft tyres.
"I would say first impressions are quite positive," he said of the car. "Everything seems to respond as we would expect.
"we only made a few small modifications to the set-up, trying to improve it, but it's obviously too early to draw any conclusions."
Ferrari's Brazilian Felipe Massa, under intense pressure to deliver after scoring just two points in five races, was an encouraging third in the second session.
Mexican Sergio Perez, returning to the scene of his big accident in qualifying last year, had been third in the morning for Sauber with race favourite Lewis Hamilton fourth for McLaren.
The largely incident-free first session was red-flagged with just over nine minutes to go when Kovalainen's car blew its engine and filled the tunnel - the fastest point of the circuit - with smoke.
Kovalainen parked up at the tunnel exit as drivers threaded their way through the smoke. With oil also deposited on the track, organisers decided not to re-start the session.
That left 2007 champion Raikkonen at the bottom of the timing screens after the Finn completed an installation lap but failed to set a time. He sat out the rest of the session as mechanics worked on his car.
"The steering wasn't to my liking so the team changed it for me," said Raikkonen. "It's something you change for Monaco and there's no way of knowing what it will be like beforehand."
Venezuelan Pastor Maldonado, surprise winner for Williams at the last race in Spain but now among the favourites at a track he loves, was again among the frontrunners with the fifth fastest time in both sessions.
Red Bull's double world champion and last year's winner Sebastian Vettel, joint championship leader with Alonso, clocked the ninth and 10th best times respectively.
Five-times Monaco winner and seven-times world champion Michael Schumacher damaged his Mercedes's front wing when he rode the kerbs at the swimming pool complex in the morning. He was 11th then and ninth in the afternoon.