Lewis Hamilton gave his new McLaren the seal of approval on Wednesday after getting back behind the wheel for the first time since he won the title in November 2008.
However the 24-year-old Briton, who completed 81 laps in the MP4-24 at the Algarve circuit in southern Portugal, warned that his team had its work cut out ahead of the season-opener on March 29.
Hamilton suffered an early setback before lunch when the car stopped on the damp circuit and had to be brought back on a truck.
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EMBARRASSING SILENCE: McLaren crew recover Lewis Hamilton's MP4-24 after it stopped on the circuit at Portimao during testing. Image: AFP
The team said the car had experienced a "precautionary engine systems shutdown" and Hamilton returned to the track in the afternoon to set the third fastest time
Rosberg was the quickest of those in the new cars
Hamilton's former team mate Fernando Alonso did 86 laps in the new Renault but was slowest of the five drivers.
Williams driver Nico Rosberg was the quickest of those in the new cars, behind 20-year-old Swiss rookie Sebastien Buemi in a 2008 Toro Rosso.
Williams technical director Sam Michael said the Toyota-powered FW31 had shown good reliability, completing more than 670km on its third day on the track .
Timo Glock's first outing in the new Toyota TF109 suffered a setback when an oil pipe became disconnected and set the car on fire.
Robert Kubica had an untroubled day testing the BMW F1.09 in Valencia, eastern Spain, while Felipe Massa completed 104 laps for Ferrari at Mugello in Italy. - Reuters