Sebastian Vettel left it late to top the lap times during the final session of free practice for the Spanish Grand Prix as Williams and Sauber both displayed impressive qualifying pace.
The championship leader dialled in a best effort of 1:23.168 with his final lap, set on the soft compound tyres, to head the timesheets from Pastor Maldonado and Kamui Kobayashi on a steadily warming Circuit de Catalunya. It sounded a warning to Vettel's rivals before qualifying as he managed the time despite sitting out a large portion of the session as mechanics tinkered with his car.
Mark Webber was fourth quickest with Sergio Perez fifth having been quickest for the majority of the session. What will be equally encouraging for Sauber, however, is the long-run pace displayed by Perez which was on a par with Nico Rosberg's Mercedes.
With teams opting to save their allocation of the more effective softer compound for the end of the session and the track yet to get up to optimum temperature, cars flooded out on the harder Pirellis.
McLaren, keeping with its new high nose configuration, left it until nearly ten minutes into the session to venture out onto the circuit to complete their hard tyre installation laps. After impressing in Friday's free practice, Lotus started the day badly, Romain Grosjean stopping at turn ten after a loss of fuel pressure caused the engine to stall. It meant the end of the session for the Frenchman and a race for the Lotus mechanics to repair the car in time for qualifying.
Rosberg was the first driver to post a meaningful time, clocking a 1:25.158 twenty minutes in. Friday morning pace setter Fernando Alonso waited until mid way through the session, the home favourite going third quickest as Rosberg became the first driver to switch to the softer compound tyre to practice a qualifying run. As expected, it had the required effect, the Mercedes jumping immediately to the top of the times.
With the track temperature steadily rising and the majority switching to the softer compounds, so the lap times tumbled, Perez setting a time just shy of Button's best mark of Friday. There he stayed until the final couple of minutes before first his team-mate then Maldonado, Webber and Vettel bettered his mark.
While the top of the timesheets has an unusual look, McLaren and Lotus displayed the pace that suggests they too will challenge for pole this afternoon. Button, Kimi Raikkonen and Lewis Hamilton all had the best laps affected, with Hamilton having already set the two fastest first sectors before being baulked by traffic at the chicane.
The session highlighted how important it will be for drivers to find clean air in Q1 and get the tyres working for a full lap later in qualifying as the field remains so closely matched.