Red Bull's Sebastian Vettel beat Ferrari's Fernando Alonso to set the pace in an incident-packed Friday practice at the Singapore Grand Prix.
Vettel, who can win the world title on Sunday if results go his way, was 0.201 seconds quicker than Alonso, who beat him into second place here last year.
McLaren's Lewis Hamilton was fastest in first practice but third in the second.
Hamilton's team-mate Jenson Button missed half of the afternoon session after going off the track.
Button locked up his brakes heading into Turn 14 and had to take to the escape road after 40 of the 90 minutes.
The 2009 world champion was advised by his race engineer that he could engage reverse gear but he replied: "Not working", before climbing out and walking away, clearly annoyed at the turn of events.
"I locked up into Turn 14 and just kept out of the barriers and selected reverse and started to go back slowly and suddenly had no drive in reverse," said Button.
"I tried to turn the car around but there wasn't enough room, then I tried to go backwards again and there was no reverse.
"So I'm guessing that something's not working correctly.
"I only ran the prime tyre and didn't have a great set-up, so it was disappointing to not do more running because I couldn't improve it.
"We have some good ideas for what we can do with the car tomorrow.
"Sebastian and Red Bull are very fast, and Ferrari are very fast over a long run as well.
"Looking at the TV and the times, everyone is going to struggle with tyres in the race. It seems everyone is overheating tyres after only three laps.
"You'd say the tyres are maybe a little bit too soft around here, but then again you don't have the grip on lap one.
"It's going to be a tough race for everyone."
Another man to push beyond the limits of the demanding, bumpy street track here was Toro Rosso's Sebastien Buemi.
The Swiss ran wide at the exit of the final chicane before the final two corners on his run on the faster super-soft tyre, and clouted the wall, taking off his right front wheel and suspension.
Hamilton and Vettel swapped fastest times in the first session, with the Englishman emerging 0.406secs ahead.
Vettel's team-mate Mark Webber was third fastest ahead of Alonso.
In the second session, Vettel set the pace from the outset, and after 40 minutes the German was a second faster than anyone else - with team-mate Mark Webber his closest pursuer, and Hamilton third.
At that time, Vettel was a massive 0.7secs ahead in the final section, which comprises seven mostly right-angled corners and the final two corners, taken as one double-apex left-hander.
Emphasising Vettel's superiority, at one point Hamilton came on the radio to this team to ask: "Where are we?" He was told: "Red Bull are 1.6secs quicker than us - most of the time to find is in the final sector."
That margin was reduced later on in the session, but Vettel and Red Bull appear to have an advantage over their rivals, particularly on single-lap qualifying pace.
However, Alonso's pace on his race-simulation run with a heavy tank of fuel late in the session was highly impressive and appeared at first glance to be faster than Vettel's.
"We are more or less happy with the car both sessions, but here it is difficult to exploit the potential of the car and there are only one or two laps when the car gives you maximum performance," said Alonso, a two-time winner in Singapore.
"Plus there is the traffic and in this situation there were red and yellow flags. So it was a mix of many things happening and during the last [session] I think maybe the times can be a little bit mixed."
Vettel said he had not been happy with the car in the first session, but that the team had improved it for the afternoon.
Hamilton, who won in Singapore in 2008 after starting on pole, said: "Tough. We were competitive in P1 and then P2 there was a big gap between us and Sebastian.
"We feel like we were doing pretty well, but we're struggling with the rear tyres on the long runs, they're dropping off very quickly on the long runs.
"These tyres, this track, you need very good downforce. We've got good downforce. I don't know what the Ferraris were running; they were massively quick. We don't know how light they were going.
"I didn't get the first lap on my options, and when I came to my second attempt the rears had given up.
"It's difficult for people to understand but if you've ever done those computer games with the rallying and you're sliding, that's what happens with these tyres. You get to a point where they don't work any more."
Webber was clearly concerned to be 0.9secs a lap off his team-mate on headline times, although he was only 0.3secs behind on heavy fuel.
He said there was some "low-hanging fruit that we can get from the car overnight" in terms of set-up improvements.
The first session was delayed twice by problems with the circuit's kerbs, which came loose at several points around the track.
The session started 30 minutes later and was shortened to an hour after loose kerbs were found following a support race, and a recurrence caused a five-minute stoppage towards the end of the session, too.
Governing body the FIA removed the kerbs on the apex of Turn 13 in a successful effort to prevent the problem recurring in the second session.
However, race officials have admitted that they have work to do overnight to ensure the kerbs stay in place during qualifying and the race.
Although his rivals are resigned to the fact that Vettel will win the title sooner or later, Ferrari and McLaren are still determined to beat him as often as possible in the remaining six races.
McLaren have a new rear wing here, and Ferrari a new front wing as they continue to develop their cars.
McLaren team principal Martin Whitmarsh said: "We enjoy winning and I think we can be competitive, we have some new parts on the car, we have six races left and we'd like to win them.
"Both the drivers are motivated and I think we can do a decent job.
"Lewis is quite relaxed and I think he's very determined. He has had a difficult season, and he did everything he could to try to stay out of trouble at the last race.
"I told him that was a very disciplined drive but I would like the old Lewis back."