Form guide: it's Vettel vs Rosberg if it rains

All we really know from extensive wet running throughout Friday in Brazil is that Nico Rosberg and Sebastian Vettel are very comfortable in their respective Mercedes and Red Bull in such conditions. With a strong possibility of rain on both Saturday and Sunday, this may turn out to be highly relevant.

Rosberg headed both sessions but it was the second that was probably more impressive in that he shaded Vettel when running on the same tyre at the same time. Rosberg's 1m27.3s on intermediates shaded Vettel by a couple of tenths, with both out there for the last half-dozen laps of a track that was improving only slightly.

"I have higher hopes now that perhaps we can give Red Bull a hard time this weekend," said Nico. "We all know that the weather plays always a big role here at Interlagos and can catch you out.

"We were not able to run that much but the track feels really great and it was great fun to be out there and find the limit in these difficult conditions."

Given that Mark Webber was running that part of the session without KERS he too was right in that ballpark and must still be considered a contender for victory in his final grand prix. Webber's programme in fact was quite extensive, as he tried to work through a full understanding of the tyre options that could be facing him tomorrow and Sunday.

"We were working on understanding the tyres in both conditions today," he reported. "We had the full wet and intermediate tyres on the car and did quite a few laps considering most people only used one set of inters.

Hamiliton could not make sense of the conditions © LAT

"It was a productive day, as we expect some more wet conditions. We have to understand the inters on both long and short runs and also try to save tyres, so it was a testing day for the teams, but overall we got what we could out of the day."

Although both Red Bull drivers were happy and looked competitive, at Mercedes Lewis Hamilton could not make sense of his car in these conditions and trailed Rosberg's best by 0.8s.

"Looking back at my career, in the rain that was when I could really make the difference, but in this car there is no hope for me," he said. "I don't know what's going on. I'm just really, really struggling with the throttle mapping and all these kinds of things.

"Clearly, the car is quick enough because of what Nico was able to do so I really need to work hard. I'm just struggling to keep it on the track. It was the same in [the similarly wet practice of] Australia."

Lotus, which in the second half of the season has been the Red Bull's closest match, typically does not respond well to cool or damp conditions and it appeared only semi-competitive here. Heikki Kovalainen was on track later than Romain Grosjean so benefitted from slightly better track conditions, his best 0.7s faster and fourth quickest overall.

Ferrari appeared to be struggling in these conditions, Felipe Massa and Fernando Alonso just seventh and 11th.

Filling in the gaps between the well spread out big teams were the predictable over-achievers in the wet: Nico Hulkenberg's Sauber and Jean-Eric Vergne's Toro Rosso, though the former was unhappy with the balance of his car in these conditions and reckoned there was much room for improvement.

There is a slight possibility that qualifying or race might be held in the dry, in which case we're flying blind. But more realistically, it's looking like Red Bull versus Rosberg.