Sauber's Kamui Kobayashi and Lotus Renault's Romain Grosjean have set identical times at the second day of testing in Mugello, Italy.
Both men posted a time of one minute, 21.603 seconds to finish ahead of Red Bull duo Sebastian Vettel and Mark Webber.
The testing opportunity in Mugello is the first time in four years that teams have been able to see if their upgrades are up to scratch in mid-season.
The first day of testing on Tuesday was virtually a wash-out because of heavy rain.
