Audi newcomer Lucas di Grassi ended up fastest on the first day of official testing ahead of this weekend's Sebring 12 Hours.
The Brazilian, who has been signed by Audi for 2013 after a starring performance on a one-off guest outing at last year's Sao Paulo World Endurance Championship round, headed the timesheets on a 1m46.262s on his first acquaintance with the Sebring International Raceway.
Di Grassi is sharing his Audi R18 e-tron quattro with Allan McNish and Tom Kristensen this weekend.
"The track is very different than what we are used to in Europe; it is like a street track on a road course," said di Grassi. "There is not a lot of margin for error.
"The car is in good shape but we have some fine-tuning to do. Being fastest on the first day means we brought a good car for this race."
The second Audi R18, which will be shared by World Endurance champions Benoit Treluyer and Marcel Fassler and Oliver Jarvis, did not take part in either test session.
The lead Rebellion Racing Lola-Toyota B12/60 shared by Neel Jani, Nick Heidfeld and Nicolas Prost ended the day second fastest, just two tenths behind with a 1m46.461s lap that topped the morning session.
The Pickett Racing HPD ARX-03c, in which Klaus Graf and Lucas Luhr are joined by Porsche factory driver Romain Dumas, ended up third on 1m47.157s.
Greaves Motorsport's Zytek-Nissan Z11SN was the only car to set a time in the LMP2 class.
The Chevrolet Corvette C6.R, driven by Oliver Gavin, Tommy Milner and Richard Westbrook, finished the day fastest in the GT class by half a second from the lead Aston Martin Vantage GTE in which Bruno Senna is making his debut with Darren Turner and Stefan Mucke.
There are a further two test sessions on Tuesday and one on Wednesday before regular practice starts on Thursday.