Red Bull Renault driver Sebastian Vettel of Germany
Red Bull's Sebastian Vettel signalled Friday that his title defence is not over as he scorched to the quickest times in both opening practice sessions for the Singapore Grand Prix.
On the floodlit street circuit, damp after heavy afternoon rain, the two-time defending champion clocked 1min 50.566sec in the first stint and then lowered it to 1:48.340 in the later run.
McLaren's Jenson Button was second quickest in the later session followed by the championship leader, Fernando Alonso of Ferrari. Red Bull's Mark Webber was fourth fastest with Lewis Hamilton placing fifth.
"This is a tough circuit for drivers and cars. Overall we can be happy with today, but let's see what we do tomorrow. It's still Friday and today's results are not so conclusive," Vettel said.
"Most important is that the car seemed to work on either tyre, so we go from there. The kerbs have been changed this year, which is an improvement."
Vettel, Formula One's youngest double champion, lies fourth in the season standings and 39 points back from the pace-setter, Ferrari's Fernando Alonso, with seven races to go.
With McLaren's Hamilton and Button sharing the last three race wins between them, the pressure is on Red Bull to fix reliability issues which have plagued Vettel and kept him back in the championship race.
Force India pair Paul di Resta and Nico Hulkenberg were sixth and seventh fastest in the later run, with Mercedes' Nico Rosberg, Ferrari driver Felipe Massa and Lotus pilot Romain Grosjean rounding out the top 10.
But Alonso warned the Ferrari car was lagging compared to their last outing at Monza, where the Spaniard finished third.
"We tried various new components on the car. Some seem to be positive, others less so, but maybe in the case of the latter, we need to spend a bit more time for them to be developed as well as possible," he said.
"Overall, I would say today was positive, even if the first impression is that we are not as competitive as we were in Monza, two weeks ago. We still lack a bit of performance on tracks like this one, where you need maximum aerodynamic downforce."
The session was held up for several minutes after Bruno Senna scraped his Williams car along a barrier and left it static in a chicane, making the track impassable until a crane could lift the vehicle clear.
Sauber's Sergio Perez, second a fortnight ago in Italy, also had a mishap but avoided making contact with the barriers, while his team-mate Kamui Kobayashi span late in the session.
Ma Qinghua, who became the first Chinese driver to take part in Formula One during practice for this month's Italian Grand Prix, finished slowest of the opening practice for HRT.
With seven races to go, Alonso leads the standings by 37 points ahead of Hamilton, who has won two of the last three races. Lotus's Kimi Raikkonen and Vettel are both close behind.