"So far we can be happy," Vettel said. "We have to do our homework and improve the car. There is a lot still to come. We don't know what the weather is going to do.
"Today was incredibly hot. Tomorrow is supposed to be hot, maybe some rain. Sunday is going to be cooler as far as they say but this is Sao Paulo so it could be anything really."
Alonso added: "We have not yet found the ideal balance on the car, so there is still a lot of work to be done with the engineers going into the rest of the weekend.
"The predictions are for very different conditions from Saturday to Sunday, further reason to concentrate on the best choices when it comes to preparing the car for qualifying, but especially for the race.
"It's hard to say if we prefer rain or the dry: as I said before, it is vital to be prepared to tackle both situations. We know we have to score 14 points more than Vettel for the championship and not lose the same number to McLaren to maintain our second place in the constructors'. Our targets are clear, but also difficult to achieve."
Some of the reduced difference in lap times between Vettel and Alonso will be explained by the relatively short Interlagos lap - the leaders' times are in the one minute 14 seconds bracket, about 30 seconds slower than at the last few grands prix.
On the longer race-simulation runs, Hamilton appeared to be in the best shape, with Alonso marginally more impressive than Vettel.
"It depends on what fuel loads people were on," said Hamilton. "But we're definitely strong, just as we were in the last three or four races. But here it's so tough on the tyres. I hope that we can continue to compete with them tomorrow."
The fastest single lap time on the race runs was by Lotus's Romain Grosjean, but Ferrari's Felipe Massa was fastest overall on average on the medium tyre on which most of a dry race would be run, just under 0.1secs faster than Hamilton.
Vettel's race average was about half a second down on Hamilton's, while Alonso - using the slower hard tyre while Vettel was on the medium - was about 0.2secs faster than his championship rival.
It is impossible to read too much into those times as the teams do not reveal the amount of fuel they have on board, and 10kg (13 litres) accounts for 0.3secs in lap time.
Formula 1's great final-day showdowns
BBC F1 technical analyst Gary Anderson said: "I don't think we've seen it as close as it's going to be tomorrow. Red Bull will pull something out of it, they always do, but Lewis Hamilton does have a good car.
"Looking at his front tyres I would say they are blistered significantly. They are going to have to look at that."
Ferrari were trying out a new front wing in an attempt to do everything they can to give Alonso a chance of winning the title.
To do so, Alonso must win with Vettel lower than fourth, be second with Vettel lower than seventh or be third with Vettel lower than ninth.
Hamilton was fastest overall in the first session, using development 2013 tyres, although Vettel set the fastest time on the standard 2012 tyres.
ractice top 10
1. Lewis Hamilton - McLaren 1:14.026
2. Sebastian Vettel - Red Bull +0.274
3. Mark Webber - Red Bull +0.497
4. Felipe Massa - Ferrari +0.527
5. Fernando Alonso - Ferrari +0.566
6. Michael Schumacher - Mercedes +0.628
7. Nico Rosberg - Mercedes +0.643
8. Jenson Button - McLaren +0.837
9. Romain Grosjean - Lotus +0.968
10. Paul di Resta - Force India +1.103
Brazilian Grand Prix, day two
Saturday, 24 November: Final practice: 13:00 GMT, 12:55 on Red Button, 5 live Sports Extra and online. Qualifying: 16:00 GMT, 15:00 on BBC One, 16:25 on 5 live sports extra and online.
Brazilian Grand Prix, day three
Sunday, 25 November: Race: 16:00 GMT, 15:00 BBC One, 15:45 5 live Sports Extra and online. Race highlights: 19:00 GMT, BBC Three.
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