Sergio Perez took advantage of drying conditions to set the fastest lap of the opening practice session at the Italian Grand Prix.

Heavy thunderstorms in the morning had led to flooding in some parts of the Monza paddock and pit-lane but after an inspection, the FIA gave the go-ahead for drivers to head out onto the circuit.

Immediately, Daniel Ricciardo and Nico Hulkenberg headed out on full wet tyres but the Red Bull driver, running the new Spec 3 engine from the French manufacturer, would slow down through the Lesmo bends as a software issue triggered a safe mode.

After a quick reset, the Australian was able to head back out and ran uninterrupted for the remainder of the session.

Despite the very wet conditions with standing water in many places, Kimi Raikkonen opted to head out on intermediates much earlier than many expected as lap times matched the 1m35s that Lewis Hamilton set for pole in the rain last year.

Max Verstappen is a driver expected to excel in the wet but would have a half spin at Parabolica as he too headed out on intermediates just as another shower passed over the circuit.

Despite forecasts for more heavy rain, it would ease with just a few drops allowing for the track to dry out, allowing for much more running than expected.

A notable absentee though would be Sebastian Vettel as a gearbox problem left the German stuck in the garage after just four laps.

Around halfway through, the Mercedes' headed out and showed the same advantage in the wet they have shown at the last three races by heading to the top of the timesheets with Valtteri Bottas ahead of Hamilton.

That would be the only run the pair would have, however, completing eight and six laps respectively.

With a drier line appearing, it was Verstappen who jumped to the front ahead of the two Renault's as Carlos Sainz led Nico Hulkenberg.

There was then shades of 2008 as the two Toro Rosso's moved to the front with Brendon Hartley fastest and Pierre Gasly third, just behind the man he'll be replacing next season, Daniel Ricciardo.

As they returned to the pits, the final minutes would then see Kimi Raikkonen and the two Force India's trade fastest laps.

It would appear the Finn had the upper hand but only a mistake at Parabolica saw Esteban Ocon finish 0.043s behind in what would become third.

That's because Perez with the last lap of the morning posted a 1m34.000s to jump over half a second clear of the field.

A full look at the times from Practice 1 can be seen below: