Sergio Perez has admitted that a day of wet-weather running at the Autodromo Jose Carlos Pace has made it difficult to know the exact potential of McLaren's MP4-28.

The car has been a handful for both Perez and team-mate Jenson Button all season, but the Briton suggested that McLaren could be a podium contender at Interlagos if everything went to plan. He started well, with a top four place in opening practice for Sunday's Brazilian Grand Prix, but spent much of the afternoon session on the sidelines, waiting for the weather to abate.

Perez, likewise, completed only eight laps after lunch, languishing down the order after filling seventh spot in the opening 90-minute session, and admitted that it was hard to know what to take from the day.

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"It's very difficult to know accurately how our car is performing - I didn't do too many laps in either session, and those that I did were done in different conditions from our rivals," he claimed.

"I'm not perfectly happy with the set-up - but at least, by doing some running, we have an understanding of what we can change for tomorrow. If the conditions stay the same, I think we'll take a step forward. If the weather stays dry, we know where we'll probably end up but, if it's wet, we could have a stronger race. Our current forecast says it'll be wet for qualifying, and that it's 50:50 for the race, but you just never know in Sao Paulo..."

Button completed 17 laps in the morning session and suggested that he had a direction for his car, even if the conditions were far from pleasant.

"It was very slippery out there today - we even had high-speed aquaplaning along the straights, which was unusual," he commented, "In these conditions, it would've been nice to have more tyres to play with - the wet weather presents more of an opportunity for us, but it's also more of a risk, so we'll see how the rest of the weekend pans out. Typically, the forecast suggests that it will be unpredictable for the remainder of the weekend.

"Fortunately, we got a lot of miles under our belts in this morning's session - we felt the conditions were consistent enough for us to conduct some running on the intermediate tyre. We did a longer run on high fuel, and also did a single-lap run on lower fuel - on each occasion, the car seemed okay. The afternoon session was more about waiting for the weather to settle down enough for us to continue running on inters. That would enable us to back-to-back our morning run in order to measure the changes we'd made between sessions. We got some useful information."

Contrary to Button's assertion, however, team boss Martin Whitmarsh tended to side with Perez, admitting that there was more work to be done in Saturday's morning practice.

"While we were able to complete a useful number of laps - particularly during the lighter rain that fell this morning - we still don't have a conclusive read on pace and performance ahead of Sunday's race," he insisted, "Still, I'm sure our engineers were kept busy collecting data during both sessions, and we'll put that information to good use tonight as we leave no stone unturned in our efforts to refine our cars' set-up for qualifying tomorrow."