Lewis Hamilton used the non-F-duct fitted McLaren to narrowly beat Sebastian Vettel and Fernando Alonso to the fastest time in a fiercely competitive final practice session at Monza, which saw the top five cars covered by just 0.226 seconds.
Felipe Massa and Jenson Button completed that lead quintet, but there was more bad news for title contender Mark Webber, whose car ground to a halt after nine laps, leaving him seventh.
McLaren dominated much of the hour. Hamilton had been the quicker of the duo after their first runs, but Button, running with the F-duct, turned the tables when they next came out - helped in part by his team-mate getting baulked by Tonio Liuzzi's Force India on the run towards Ascari.
Button then trimmed his time from 1m22.858s to 1m22.768s on the softer tyres with six minutes to go, before Alonso delighted the home crowd by putting Ferrari on top with first a 1m22.656s, then an improvement to 1m22.644s next time around.
Several drivers beat Alonso's pace in the first two sectors then lost out either to traffic or the Spaniard's superior pace through the Parabolica.
But with just under two minutes to go, Hamilton put a lap together, grabbed a useful tow from Adrian Sutil's Force India on the back straight, and deposed Alonso with a 1m22.498s.
The Ferrari driver's hopes of responding were dash when Alonso encountered Kamui Kobayashi's Sauber halfway round the Parabolica, prompting some fist-waving from the double champion, and in the end Alonso was pushed back to third as Vettel slipped up to second with an excellent last-gasp lap.
Massa recovered from a high-speed visit to the Lesmo gravel to take fourth ahead of Button, with Nico Rosberg sixth in the lead Mercedes.
Webber's session came to an early end when his Red Bull suddenly lost power and shut down at the first chicane - meaning the Australian has had mechanical problems in two out of three practice sessions this weekend.
Williams had a strong session, with Nico Hulkenberg eight and Rubens Barrichello 11th, split by Robert Kubica (Renault) and Sutil.
The only other incident of note was Jaime Alguersuari (Toro Rosso) emulating Massa's Lesmo mistake. Like the Brazilian, he stayed away from the barriers.
Pos Driver Team Time Laps
1. Hamilton McLaren-Mercedes 1:22.498 19
2. Vettel Red Bull-Renault 1:22.545 + 0.047 21
3. Alonso Ferrari 1:22.644 + 0.146 15
4. Massa Ferrari 1:22.648 + 0.150 16
5. Button McLaren-Mercedes 1:22.724 + 0.226 19
6. Rosberg Mercedes 1:22.946 + 0.448 17
7. Webber Red Bull-Renault 1:23.082 + 0.584 9
8. Hulkenberg Williams-Cosworth 1:23.129 + 0.631 18
9. Kubica Renault 1:23.209 + 0.711 20
10. Sutil Force India-Mercedes 1:23.303 + 0.805 19
11. Barrichello Williams-Cosworth 1:23.450 + 0.952 19
12. Buemi Toro Rosso-Ferrari 1:23.673 + 1.175 20
13. Schumacher Mercedes 1:23.896 + 1.398 21
14. Kobayashi Sauber-Ferrari 1:23.908 + 1.410 24
15. Alguersuari Toro Rosso-Ferrari 1:23.909 + 1.411 17
16. Petrov Renault 1:23.967 + 1.469 19
17. de la Rosa Sauber-Ferrari 1:24.191 + 1.693 19
18. Liuzzi Force India-Mercedes 1:24.439 + 1.941 15
19. Trulli Lotus-Cosworth 1:25.788 + 3.290 14
20. Kovalainen Lotus-Cosworth 1:25.925 + 3.427 16
21. Glock Virgin-Cosworth 1:26.434 + 3.936 18
22. di Grassi Virgin-Cosworth 1:26.682 + 4.184 19
23. Senna HRT-Cosworth 1:27.471 + 4.973 18
24. Yamamoto HRT-Cosworth 1:28.730 + 6.232 14
All Timing Unofficial