Renault's Fernando Alonso set the fastest time of the weekend so far in the final moments of the final free practice session in Monaco on Saturday morning as the top seven runners were covered by less than 0.3 second.
In a frantic and closely-fought hour, the 2005 and '06 World Champion eclipsed this year's points leader Jenson Button by just 0.069s with his 1m15.233s, while Heikki Kovalainen's final effort of the session put a McLaren third in the order.
Rubens Barrichello was fourth in the second Brawn ahead of the Ferraris of Felipe Massa and Kimi Raikkonen. Lewis Hamilton was seventh, just 0.225s behind Alonso.
Williams driver Nico Rosberg was ninth, sandwiched between Red Bull drivers Sebastian Vettel and Mark Webber.
The Toyotas of Jarno Trulli and Timo Glock were the early pace-setters in the session, but only until Hamilton got into his stride with quarter of an hour gone.
The World Champion then held sway for a large portion of the session despite stiff opposition in the first 20 minutes from Alonso and Rosberg, as the McLaren driver reeled off a sequence of five consecutive flying laps - only one of which was not an improvement when he encountered Robert Kubica's slow BMW at the chicane.
Hamilton's 1m15.622 was good enough to keep him comfortably at the top of times until the teams began switching to the super-soft Bridgestones in the final 20 minutes.
But even then however, it was Hamilton that was the first to improve on his own mark with a 1m15.389s. Though on his next lap he was forced to pit after damaging his front wing against the wall.
Behind him the Ferraris of Raikkonen and Massa engaged in a battle for second with Barrichello. And eventually, with 12 minutes to go, Ferrari's 2007 world champion finally moved ahead of Hamilton with a lap of 1m15.382s - just 0.007s ahead of the Briton.
Button meanwhile had been quietly working through his programme until he too bolted on a set of the green-rimmed option tyres and suddenly the Brawn was the fastest car on the track, outpacing the Ferrari by 0.149s to go top.
But the inevitable flurry of activity in the final minutes changed the order again and it was in those final frantic minutes, as frontrunner after frontrunner encountered the inevitable traffic problems, that Alonso emerged fastest with a 1m15.164s.
After three sessions there has still to be any significant incidents to report, with Giancarlo Fisichella the only spinner when the Force India gyrated harmlessly at the Nouvelle Chicane.
Pos Driver Team Time Laps
1. Alonso Renault (B) 1:15.164 24
2. Button Brawn-Mercedes (B) 1:15.233 + 0.069 29
3. Kovalainen McLaren-Mercedes (B) 1:15.278 + 0.114 24
4. Barrichello Brawn-Mercedes (B) 1:15.286 + 0.122 26
5. Massa Ferrari (B) 1:15.293 + 0.129 23
6. Raikkonen Ferrari (B) 1:15.382 + 0.218 25
7. Hamilton McLaren-Mercedes (B) 1:15.389 + 0.225 23
8. Vettel Red Bull-Renault (B) 1:15.722 + 0.558 23
9. Rosberg Williams-Toyota (B) 1:15.758 + 0.594 23
10. Webber Red Bull-Renault (B) 1:15.985 + 0.821 24
11. Nakajima Williams-Toyota (B) 1:16.103 + 0.939 22
12. Sutil Force India-Mercedes (B) 1:16.228 + 1.064 20
13. Bourdais Toro Rosso-Ferrari (B) 1:16.301 + 1.137 23
14. Fisichella Force India-Mercedes (B) 1:16.317 + 1.153 29
15. Piquet Renault (B) 1:16.382 + 1.218 27
16. Buemi Toro Rosso-Ferrari (B) 1:16.432 + 1.268 22
17. Glock Toyota (B) 1:16.527 + 1.363 29
18. Kubica BMW Sauber (B) 1:16.599 + 1.435 26
19. Heidfeld BMW Sauber (B) 1:16.661 + 1.497 22
20. Trulli Toyota (B) 1:16.810 + 1.646 26
All timing unofficial