After an incident-filled start at a damp Monza, who will make the running in the second session? Watch live on Sky Sports F1.
Kimi Raikkonen has just clattered over the yellow kerbs at the first chicane as his day resumes following that morning incident at Parabolica.
It's crazy to see so many cars in a train in practice, with every driver ,looking for the tow down the straight. Most of the 19 drivers out on track are in tandem.
Sebastian Vettel and Max Verstappen are now up to second and third, behind Hamilton who has improved to a 1:21.972.
Verstappen is told that rain is expected in the next 15-20 minutes.
"So that explains why everyone is out at the moment," says Crofty.
Lewis lonely on track - but still fastest "It looks like you forgot me," Hamilton tells his race engineer.
"I'm out here on my own. There's no one to tow."
No tow, or no one to tow, but Hamilton has taken the early timesheet lead.
An incredibly busy start to this session. Lewis Hamilton and Charles Leclerc are straight up into first in second after four laps apiece.
Hamilton currently leads with a 1:22.450, though Leclerc is just four hundredths back. Valtteri Bottas is 0.078s slower in third.
"Leclerc is the star turn in the paddock here.
"He just got absolutely mobbed while he walked through."
Leclerc and Vettel now out in tandem on circuit - some tow practice ahead?
Back underway at Monza - with the cars straight out on dry tyres this time.
"We had rain this morning this morning and overnight but the track is finally dry," says Sky F1's David Croft ahead of second practice.
There is, however, an 80 per cent chance of rain in this session.
"They have to make use of this dry track," adds Anthony Davidson.
A wasted opening session for Kimi Raikkonen after spinning into the gravel at Parabolica, but his Alfa Romeo should be dusted down and ready to go for this second session.
Who has new engines this weekend? Max Verstappen, Pierre Gasly: New ICE, TC, MGU-H, ES, MGU-K, CE
Lando Norris: New ICE, TC, MGU-H, ES, MGU-K
Sebastian Vettel, Charles Leclerc: New ICE, TC, MGU-H
In Verstappen, Gasly and Norris' case - they will start at the back of the grid due to those changes.
Ferrari take no penalties and that new engine, their second upgrade of the season, has been timed for the fastest track on the calendar at Monza.
Ferrari the favourites?"Mercedes were looking very strong in those changeable conditions before it dried, but if ever there was a Monza-spec car for the dry it's the Ferrari this year.
"The only thing that might hamper them is whether they have enough downforce to turn the tyres on. But Ferrari are happy and in good spirits."
P1 sessions don't often come as action-packed as this morning's did at Monza. Anthony Davidson is at the SkyPad to analyse all the key incidents...
No rain since P1 After heavy rain threatened to wipe out first practice completely we actually had an incident-filled session, with three big spins and plenty of changes in the leaderboard in the closing stages.
And the good news is that we've had no rain since then - which means drivers should be on slicks at the start of P2. So we should get a proper look at the pecking order.
Charles Leclerc will hope to build on his session-topping form from this morning.
Practice Two underway at dry Monza; Charles Leclerc fastest in drying P1 session after morning rain; Sergio Perez and Kimi Raikkonen spun out; Get Sky F1's limited time £10 offer
"It looks like you forgot me," Hamilton tells his race engineer.
Max Verstappen, Pierre Gasly: New ICE, TC, MGU-H, ES, MGU-K, CE
After heavy rain threatened to wipe out first practice completely we actually had an incident-filled session, with three big spins and plenty of changes in the leaderboard in the closing stages.