Heidfeld, who is out of contract at the end of this season, set a best time of 1:22.621 as he focused on set-up work ahead of the Italian GP, which takes place at the Monza circuit.
Second place went to Williams’ Nico Rosberg, who finished a quarter of a second off the pace, while Heikki Kovalainen completed the top three.
Toro Rosso’s Sebastian Vettel was fourth fastest ahead of Ferrari’s Felipe Massa who had been quickest on day one.
Renault’s Fernando Alonso was sixth fastest ahead of Adrian Sutil (Force India), Mark Webber (Red Bull), Jarno Trulli (Toyota), Sebastien Bourdais (Toro Rosso) and the Honda duo of Rubens Barrichello and Jenson Button.
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