Fernando Alonso admits that the weather could be the key factor in the Brazilian GP weekend after he set the pace during the opening day of running at Interlagos.

Both Friday sessions were affected by the weather, with Alonso one of a number of drivers to suffer a spin when rain fell during FP1.

However, in better conditions in the second session, the Renault man was able to edge out Sebastien Buemi to set the quickest time although the Spaniard conceded that the conditions hadn't made things easy - and wouldn't do during the weekend unless the team was able to react quickly to any changes that occur.

"Today wasn't easy because the weather conditions were changing all the time," he said. "That made it difficult to concentrate on the set-up of the car, but in the end we did some good runs with positive results and hopefully tomorrow we can continue this good work.

"I think that the weather will be the key to this weekend so we need to react quickly to any changes and be flexible. We have a very interesting weekend ahead of us and hopefully we can maximise the work that we do tomorrow morning so that we are in good shape for qualifying."

Team-mate Romain Grosjean brought out the red flags during the first session when he ran wide and destroyed a marker on the side of the circuit, but the rookie bounced back to set the eleventh quickest time during the second session of the day.

"The day has gone quite well but the weather has been so unpredictable," he said. "We still managed to make made good progress and tomorrow morning I will continue working hard to improve my times. This was my first experience of the Interlagos circuit and it's a great place, although it isn't easy and the track has lots of uphill and downhill sections which are challenging. Tomorrow I need to continue learning the track as well as completing my preparations for qualifying and the race on Sunday."