Sebastien Buemi kept Scuderia Toro Rosso at the top of the times for the third day in a row at Portimao on Wednesday, as the wet weather that had plagued proceedings finally abated.
Buemi, who was once again the only man running in a 2008-spec car, managed 128 laps in total, with STR cancelling its lunch break to get in as much dry weather running as possible.
The track remained damp in the morning, but from around mid-day it was possible to fit slick tyres and Buemi made the most of it to begin 'serious work', concentrating mainly on suspension systems. His best lap for the day was a 1 minute 27.987 seconds.
Nico Rosberg meanwhile was 'best of the rest' and the fastest of those in the 2009 cars. His quickest effort was a 1m 29.729s and he did 143 tours in total – easily the most of any one runner – while focusing on initial systems, performance and reliability checks.
"We had a wet track for the first couple of hours in Portimao today, but by late morning it had dried out enabling us to run slick tyres on the FW31 for the first time," explained Williams F1 technical director, Sam Michael.
"It was a constructive day as we worked through a lot of the items on our job list, including systems checks, aerodynamic and mechanical performance tests. The FW31 reliably completed just over 670kms today on only its third day on track. Kazuki [Nakajima] will take over from Nico for the last day of testing tomorrow and for his first run in the new car."
Lewis Hamilton was next up for McLaren, around half a second off Rosberg's Williams, and getting his first taste of the new MP4-24.
Like the other runners, the Brit benefitted from the improved weather conditions and this allowed McLaren to fix the 09-spec rear wing in the afternoon, when he set his best lap, a 1m 30.242s.
"It was good to be back working with the team," said the reigning champion, "it felt like I'd hardly been away because it was easy to slip back into the routine. It's amazing to think that, while I've been to Woking throughout the winter, I haven't properly driven a Formula 1 car since the Brazilian Grand Prix two months ago, but everything very quickly felt normal today.