By John O'Brien
SINGAPORE (Reuters) - Nico Rosberg clocked the fastest time of the day in the second free practice session for the Singapore Grand Prix on Friday in a race that appears to be shaping up as a three-way battle between Mercedes, Red Bull and Ferrari.
The German, who will start his 200th Formula One grand prix on Sunday, lapped the 23-turn Marina Bay Street Circuit in one minute 44.152 seconds to edge out Ferrari's Kimi Raikkonen and Red Bull's Max Verstappen.
The top three finishers were separated by just 0.380 seconds on the timesheets as Rosberg's Mercedes was unable to fully tap into its superior power on the slow, tight circuit in downtown Singapore.
Rosberg has won the last two races to close the gap on championship-leading team mate Lewis Hamilton to two points and the Briton cut a frustrated figure on Friday, finishing seventh in the second session after suffering a hydraulics issue.
Mercedes have won 13 of 14 races this season but the race pace during Friday's long runs would suggest a repeat of last year's result when Ferrari's Sebastian Vettel led home Red Bull's Daniel Ricciardo is entirely possible.
Australia's Ricciardo was fourth fastest on Friday, ahead of Vettel and Nico Hulkenberg in a Force India.
In the first session of the day, Verstappen and Ricciardo achieved a Red Bull one-two as the 18-year-old Dutchman lapped in one minute 45.823 seconds to head his team mate by 0.049 seconds. Vettel was next fastest, 0.464 off the pace.
Hamilton finished 0.603 seconds behind Verstappen but fifth-placed Rosberg had his session ended prematurely when he damaged his front wing after crashing late in the 90-minute session.
The Mercedes duo set their fastest times on the more durable soft compound tyres, however, with the top three all going quicker on ultrasofts.
The session started in the twilight as the cars went out for their first stints at the end of a sweltering day but McLaren's Jenson Button ground to a halt towards the end of his installation lap, bringing out the virtual safety car.
"Yes, it's stopped," the Briton lamented over the team radio. "I'm not going to make it back to the pits."
All the other cars returned to the pits as the Briton was pushed a few hundred metres to his garage by marshals and crew members.
After a mid-session lull when all the cars returned to the pits to hand back their first set of tyres, Rosberg soon had the track to himself and immediately took almost a second off Verstappen's early pace-setting time.
The Red Bulls then went out on new sets of ultrasofts to dominate the remainder of the session, with Vettel also going quicker than Mercedes on his final stint.
(Editing by Tony Jimenez and Toby Davis)