After skipping a chicane to set a fast lap earlier this week, Sauber rookie Sergio Perez on Thursday leapt to the top of the times on merit.
The Mexican was doing a full qualifying simulation on low fuel and a new set of super soft Pirellis, but world champion Sebastian Vettel was nonetheless impressed.
"Wow, that was a quick lap," the Red Bull driver is quoted as saying by Blick newspaper.
The laptime, faster than the field-leading Ferrari and Red Bull, was also the fastest time seen all year at the Circuit de Catalunya.
"I don't think I could have done that time," Force India's Adrian Sutil told Auto Motor und Sport.
The German added: "At the front are Red Bull and Ferrari. Toro Rosso look good, and Sauber have also been very fast."