Torrential rain wrecked havoc with the teams pre-qualifying preparations in Monza, leaving early pace-setter Timo Glock on the top of the time sheets.
Showers from the south of the circuit arrived just in time for the start of Saturday practice and conditions quickly deteriorated.
Rubens Barrichello and Timo Glock were among those to venture out onto the slippery track early on, the latter driver going quickest on a 1:38.261 before improving to a 1:35.464, ahead of Sebastian Vettel, Nico Rosberg, and team-mate Jarno Trulli.
As the rain got harder, so the spins and aquaplaning become more frequent. Fernando Alonso was first to fall victim to the track conditions; the Spaniard put on the extreme wet Bridgestone but they were not up to temperature and he found himself bouncing over the run-off at the second chicane.
He was followed shortly by his Renault team-mate Nelson Piquet who over-cooked it into the first chicane and scooted down the escape road to rejoin at Curva Grande.
By mid-point in the session the rain was falling heavily and the drivers were struggling to keep their cars pointing in the right direction, let alone set a quick lap.
Jenson Button lost control on the exit at the Roggia chicane, then Nico Rosberg and Robert Kubica both resorted to using the escape road at the first corner as they struggled to bring their cars down to speed for the turn-in point.
With less than a quarter of an hour remaining the vast majority of the drivers chose to sit it out in the pits for the rest of the session.
Nick Heidfeld, Lewis Hamilton and Heikki Kovalainen all made exploratory laps, snaking their way around the 3.6 mile track with spray in abundance, but they quickly returned to the pits.
That left Timo Glock quickest followed by Vettel, Rosberg, Trulli and Nakajima. Felipe Massa was eighth with his recently-confirmed team-mate for next year Kimi Raikkonen, in 18th.
Lewis Hamilton finished the session at the bottom of the timesheet for the first time in his Formula One career.