Alonso sets early Monaco pace
Fernando Alonso set the pace in a first practice session spectacularly shortened by Heikki Kovalainen in Monaco.
With nine minutes remaining of the hour-and-a-half event around the streets of the principality, the Renault engine in Kovalainen's Caterham decided to give way.
Worse still it did so as the Finn hurtled through the tunnel, filling the space under the Fairmont hotel with clouds of white smoke.
Kovalainen finally came to a halt on the short straight emerging out of the tunnel down to the Port Chicane, forcing an immediate red flag.
Given the poor visibility in the tunnel, it was soon decided to call a halt on the session at the end of which Alonso in his Ferrari emerged at the head of another congested field.
Alonso, joint leader in the standings alongside reigning champion Sebastian Vettel, finished with a lap of one minute 16.263 seconds.
Given the time there is plenty of margin for improvement bearing in mind Vettel's pole lap from last year was 2.7secs quicker.
Lotus' Romain Grosjean led the way for most and at various stages of the session, but was forced to settle for second best behind Alonso, finishing 0.365s adrift.
On a contrasting morning for Lotus, Kimi Raikkonen endured frustration as a steering issue confined him to the garage for all but one installation lap, leaving him without a time.
Sauber's Sergio Perez, back on track here a year after suffering a frightening accident that left him in hospital for two days and forced him out of two races, was third overall.
The Mexican, along with McLaren's Lewis Hamilton - now a Monaco resident - and race-winner last time out in Spain in Pastor Maldonado in his Williams, all finished just under half a second down, with the trio covered by 0.049.
Ferrari's Felipe Massa, with the pressure on him this weekend to perform, finished reasonably close to team-mate Alonso for once, the Brazilian sixth on the timesheet, 0.578 off the pace.
Kamui Kobayashi was next up in the second Sauber, followed by McLaren's Jenson Button, Vettel in his Red Bull and Mercedes' of Nico Rosberg to round out the top 10, all covered by one second.
Seven-times champion Michael Schumacher, who will serve a five-place grid penalty on Sunday for running into the back of Bruno Senna in his Williams in Spain, was 1.148 down in 11th.
Red Bull's Mark Webber, winner here two years ago, was 13th, with Force India's Paul di Resta 16th, two seconds down, and notably 0.7 off team-mate Nico Hulkenberg who was 12th.