Nico Rosberg topped yet another practice session for Williams on Thursday afternoon in Monaco, beating last year's race winner Lewis Hamilton (McLaren) to the top spot.
Rubens Barrichello, who had been quickest in the morning, took third this time, ahead of his championship-leading Brawn team-mate Jenson Button, and Felipe Massa in the best of the Ferraris.
McLaren held the advantage for much of the session, with first Heikki Kovalainen then Hamilton establishing themselves at the top following a frenetic opening 20 minutes as countless drivers swapped first place on an ever-improving track.
It was Massa who deposed Hamilton with a soft tyre run just before the one-hour mark, despite getting sideways out of Mirabeau, but he only led the way briefly before Barrichello regained the superiority he had shown this morning with a lap of 1m15.590s, half a second clear of the Ferrari.
Then, when as the pace increased further in the final five minutes, Rosberg and Hamilton thrust themselves to the front. The Williams was first to lead the session, before Hamilton beat it by 0.001s.
Rosberg retaliated though, lapping in 1m15.243s to out-pace Hamilton by 0.202s and give himself opening day bragging rights yet again. A further attempt to improve ended in the Mirabeau escape road, but it did not matter by then.
With Hamilton and Barrichello hanging on to second and third, Button eventually moved up to fourth ahead of Massa after a relatively quiet session.
Red Bull recovered from its troubled morning - which saw Sebastian Vettel stop with a blown engine and Mark Webber lose time with a transmission glitch - to take sixth with Vettel, although Webber was only 12th.
Kovalainen fell back to seventh by the end of the afternoon, followed by Kimi Raikkonen (Ferrari), Kazuki Nakajima in the second Williams, and Renault's Nelson Piquet.
Robert Kubica's involvement in the session was extremely brief. His BMW's engine erupted in a cloud of smoke and flames as he accelerated out of Sainte Devote on what should have been his first flying lap, leaving him stranded at the top of the hill and at the bottom of the timing screens.
That was the most dramatic incident in a session that again saw everyone avoid the barriers, although Massa, Vettel, Sebastien Bourdais and Adrian Sutil visited the Sainte Devote escape road and many others took the shortcut across the chicane.
Pos Driver Team Time Laps
1. Rosberg Williams-Toyota (B) 1:15.243 45
2. Hamilton McLaren-Mercedes (B) 1:15.445 + 0.202 34
3. Barrichello Brawn-Mercedes (B) 1:15.590 + 0.347 40
4. Button Brawn-Mercedes (B) 1:15.774 + 0.531 35
5. Massa Ferrari (B) 1:15.832 + 0.589 41
6. Vettel Red Bull-Renault (B) 1:15.847 + 0.604 33
7. Kovalainen McLaren-Mercedes (B) 1:15.984 + 0.741 44
8. Raikkonen Ferrari (B) 1:15.985 + 0.742 42
9. Nakajima Williams-Toyota (B) 1:16.260 + 1.017 42
10. Piquet Renault (B) 1:16.286 + 1.043 42
11. Alonso Renault (B) 1:16.552 + 1.309 38
12. Webber Red Bull-Renault (B) 1:16.579 + 1.336 27
13. Sutil Force India-Mercedes (B) 1:16.675 + 1.432 37
14. Trulli Toyota (B) 1:16.915 + 1.672 42
15. Buemi Toro Rosso-Ferrari (B) 1:16.983 + 1.740 47
16. Bourdais Toro Rosso-Ferrari (B) 1:17.052 + 1.809 47
17. Heidfeld BMW-Sauber (B) 1:17.109 + 1.866 39
18. Glock Toyota (B) 1:17.207 + 1.964 44
19. Fisichella Force India-Mercedes (B) 1:17.504 + 2.261 44
20. Kubica BMW-Sauber (B) No time 2
All Timing Unofficial