* Vettel finishes more than half a second clear
* Mercedes show improvement on high downforce circuit
By John O'Brien
SINGAPORE, Sept 20 (Reuters) - Sebastian Vettel justified his favourite tag for the Singapore Grand Prix when he stormed clear of his rivals in Friday's second free practice, lapping more than half a second quicker than Red Bull team mate Mark Webber.
The German triple world champion is 53 points ahead of Ferrari's Fernando Alonso heading into the weekend and his dominant display around the Marina Bay Street Circuit suggests he will extend that advantage in Sunday's night race.
Vettel, who is seeking a hat-trick of victories in Singapore as well as a hat-trick of wins since he finished third in Hungary in July, clocked an unchallenged one minute 44.249 seconds to leave all but Webber more than a second adrift.
The Australian, who is retiring from Formula One at the end of the season, finished 0.604 seconds off the pace with the Mercedes duo Nico Rosberg and Lewis Hamilton enjoying a return to a high downforce track by securing the next two places.
Rosberg was 1.009 seconds down on Vettel in the second 90-minute session with Hamilton marginally slower after the Briton had been quickest in the first practice session ahead of the two Red Bulls.
When asked if he expected to be so far ahead of his rivals, Vettel offered a cautious response.
"No, it was a bit of a surprise," the 26-year-old told a scrum of reporters outside the Red Bull garage.
"I think we are quick but I think Mercedes will be very, very competitive tomorrow when we go into qualifying."
Both sessions were fairly incident-free especially with the infamous 'Singapore Sling' turn 10 chicane less of a threat after track officials took heed of driver complaints.
A few drivers brushed the wall on the exit of the turn and the Williams of Pastor Maldonado was the only car to suffer any major damage when he crumpled his front wing after overshooting turn 13.
However, Vettel was virtually perfect throughout both sessions and once his Red Bull was unleashed on the super soft option tyres, there was only ever going to be one outcome on a circuit tailor made for his style and the car's strengths.
After two difficult weekends on the low downforce circuits in Belgium and Italy, Mercedes are hoping for a marked improvement on a high downforce layout.
Hamilton won in Hungary, another tight and twisty circuit, while team mate Rosberg was victorious in Monaco, the slowest of all.
The German team's confidence was quickly backed up by Hamilton in the first 90-minute session of the weekend with the 2008 world champion saying "the circuit feels twice as grippy as last year" over the team radio after setting the fastest lap.
While not lapping as quickly as the Red Bulls and with Hamilton defying a team call to change to medium prime tyres late in the second session, Mercedes will be buoyed by their performance and can expect to challenge for pole position in Saturday qualifying.
"It wasn't perfect, it wasn't the best but a normal Friday for us," Hamilton said while admitting the pace of the Red Bulls was "impressive".
"I think it (the gap) will reduce slightly perhaps but I might be wrong.
"As always every Friday night we have to try and pull a miracle out from nowhere, so we will have to do the same."
For Alonso, it was a disappointing day behind the wheel as rumours circulated around the paddock that he could be poised for a move back to McLaren.
The Spaniard failed to make an impact and was only sixth fastest in the second session, one spot higher than his earlier placing.
Alonso has won twice in Singapore but Ferrari will need to find more speed if he is make any inroads to the large deficit to Vettel.
The Singapore Grand Prix is the 13th round of the 19-race season with races in Korea, Japan, India, Abu Dhabi, the United States and Brazil to follow. (Additional reporting by Patrick Johnston; Editing by Alan Baldwin and Alison Wildey)