Germany's Nico Rosberg, who has yet to transform his practice pace into anything higher than sixth place in a race this season, put Williams on top of the timesheets at the Bahrain Grand Prix on Friday.
Rosberg, son of 1982 champion Keke, lapped the Sakhir circuit in a time of one minute 33.339 seconds. The German has now been quickest in seven of the 11 practice sessions this year.
Renault's double world champion Fernando Alonso was second fastest in the afternoon.
Current champion Lewis Hamilton had earlier shrugged off the troubles surrounding his McLaren team to clock the quickest time in the morning.
The Briton, whose team risk heavy sanctions at a hearing in Paris next week after a storm over lying to stewards at the Australian season-opener, lapped in 1:33.647.
Only Rosberg, Alonso and Toyota's Jarno Trulli went faster in the afternoon.
Red Bull's Sebastian Vettel, winner of the last race in China, was fourth fastest in the second session ahead of Australian team mate Mark Webber and Brawn GP's championship leader Jenson Button.
BMW-Sauber had filled second and third place on a hot and overcast morning with Germany's Nick Heidfeld ahead of Poland's Robert Kubica.
The top three cars in that session were all equipped with the new KERS energy recovery system, that offers drivers an extra boost of power at the push of a button, at a circuit where it is expected to be a real advantage.
Only Alonso of the afternoon's top three had KERS on board.
Button, who won the first two races of the season and finished third in China, is seen as a favourite to return to the top of the podium with the Brawn likely to be more competitive in the hot conditions.
Mercedes-powered McLaren, who warned at the start of the season that their car was still some way off becoming a winner, have brought new developments to Bahrain in an effort to close the gap at the top.
"We have a few modifications for the floor, hopefully a little bit more downforce, but nothing dramatic," said Kovalainen, whose team mate has been kept well away from the media since arriving in Bahrain.
"There is nothing major at this race. I think the next big package should be in Spain."