So Hamilton quickest again - but only by a whisker from Alonso. Will it be those two on the front row in qualifying. Join us at 13:00 UK time to find out!
Free Practice 3
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The top 10: Hamilton, Alonso, Di Resta, Massa, Button, Rosberg, Perez, Raikkonen, Hulkenberg, Maldonado.
Perez a late improver - up to 7th at the death.
SESSION OVER! Drivers finishing their laps - but it's a slightly muted end to the session after Vettel's woes.
Yellow flags out as engineers deal with Vettel's car - replays show that he had a loss of power and chose to back off - he told his pits it was a 'low battery'.
VETTEL HAS STOPPED! Not sure what's happened there, but the car is parked up by Ascari.
Button has another go at setting a new best, but finds traffic in the form of Raikkonen at the end of the lap.
Vettel a late arrival into the action. Team-mate Webber down in 12th, almost a second back, the Red Bulls looking a slight concern in qualifying.
And Hamilton does lead! By the slenderest margin possible in Formula One - 0.001 seconds. 1:24.578 is the new benchmark.
Alonso has shaved six-hundredths of a second off his leading time - so it will be just that fraction more difficult for someone to eclipse him. Hamilton doing his best to do so...
Replays show Raikkonen having a bit of a wobble in the Parabolica - just as Massa had done a minute or two earlier. Neither better their earlier times.
Button round again in a 1:25.0 - so no gain there. Jenson was complaining of understeer earlier, although frankly it wouldn't be a practice session if the McLaren man wasn't doing that.
Alonso finds himself on the back of Rosberg - but the tow actually plays into his favour, helping him set a fastest sector time in the final sector. No overall improvement, however.
Small improvement for Button - a 1:24.994 that cements his third place. Those tyres may still warm up a touch with another lap or two, just like the hard compound tyres have done.
Button, sure enough, is followed out by Hamilton, the Ferraris, and Raikkonen. Rosberg, the only other man in the top six, was already running.
Predicted gains are supposedly 0.3-0.5 seconds on the medium compound compared to the hard, the F1 pit crews are predicting. We'll see.
Button out on track - medium rubber - he's the first, and he won't be the last.
A lot of focus on Monza, but since we're in Italy it would be remiss not to mention the achievement of former F1 star Alex Zanardi, who's won Paralympics gold this week.
A slightly quiet spell here - we've not seen the big names go out on the medium compound tyres yet. We could be in for a little flurry of activity towards the end of the session.
Hamilton given some tips about where Alonso gained his speed - he is told that Alonso was "one foot outside the white line" at Ascari, running just out on to the astroturf beyond the kerb.
Vettel makes a slight improvement on the hard compound, up to 7th with a 1:25.406.
That's a bit better - the 7-times World Champion moves up to 11th and within 0.931 seconds of Alonso's time - but it's still on a different compound to the Ferrari, and it's half a second down on team-mate Rosberg to boot.
Schumacher still down in a lowly 18th despite the medium compound tyres. Not his morning so far - well off the pace.
Button shaping up to get out and put some laps down, but at the moment onl Maldonado and Vettel from the top 10 are out on track.
Whoops - D'Ambrosio makes a mess of his braking, and rides the first chicane speed bumps in his Lotus.
As it happens, Ron Dennis is in the McLaren garage today - now there's a power meeting.
Luca di Montezemolo saunters into the McLaren garage, starts greeting them all like old friends. A man who feels very comfortable in the Formula One paddock...
No such joy for Bruno Senna on the same tyres for Williams - he is held up by a Marussia, and has to run over speed bumps at the second chicane.
Rosberg jumps up to third with a 1:25.036. That's a a bit more like it.
Here's something that will shake things up a touch - however - the Mercedes are out on the medium tyres - they have been relatively anonymous this morning.
The track temperature has risen from around 32C to 37C as the morning has passed, but it doesn't seem to have made a huge difference to the speeds or the track evolution so far.
Now it's Alonso's turn to lead the standings - 1:24.632 lifts him to the top of the timesheets.
Massa up to fourth now, and it looks like finally McLaren have a serious challenger this weekend. Red Bull have yet to join the top teams, with Vettel 9th and Webber 13th.
Massa slightly eclipses that - but Alonso then rushes up to second with a 1:25.186. Di Resta and Rosberg back out on track heading for a second run.
Alonso posts a very decent 1:25.870 to start - decent because we've seen others bring that down very quickly after putting a couple more laps down.
Ferraris out on track - how will they fare?
Maldonado - occasionally reckless but usually very fast - he's up to third with a 1:25.383.
A quick happy birthday - Vitaly Petrov of Caterham turns 28 today! What the team would give to see him celebrate that with a points finish tomorrow in Italy...
Both McLarens in the pits - presumably not sharing any of the data that they have just gleaned from that running.
Hamilton improves - again! Down to a 1:24.687 - and with that he heads back into the pits presumably more than happy with his work so far.
Jerome D'Ambrosio, running for the suspended Romain Grosjean, is currently 13th - not bad given he's coming into the race weekend off minimal running time. A big opportunity for him after spending a season tailing around the back of the grid with Marussia.
No sign of Ferrari as yet - they are keeping their fans waiting. Problems for Alonso yesterday scuppered his FP2 session, so he'd presumably like a good run-out.
Top 10 so far: Hamilton, Button, Perez, Senna, Di Resta, Raikkonen, Vettel, Hulkenberg, Kobayashi, Maldonado
Sure enough, Hamilton roars through with a 1:25.165, and Button slips in a tenth of a second behind him. Early days again, but the McLarens still look very good indeed.
Perez down to 1:25.508, Raikkonen slots in behind him. Hamilton appeared to back off in lap two, perhaps to build the tyres up for lap three.
Button into sixth, Vettel begins his first timed lap of the day.
Hamilton eclipses it as well - 1:25.816 in his first effort - but already Perez and Raikkonen appear to be shaving more tenths of their times as the rubber warms up. Another weekend in which tyre performance will be key? Of course it will be.
Perez's 1:25.894 is the new leading mark - that's a bit more like it. Drivers are on hard tyres at the moment - it's mediums and hards for this weekend's racing.
Sure enough, Hulkenberg's 1:30.583 won't be worrying anyone. Monza is Formula One's fastest track - lots of room to go quicker there. McLaren are out now.
Tyre temperatures are low, Hulkenberg told - so get some running time in before you get up to optimum pace.
Here come the drivers - Hulkenberg the first out for Force Inida, Perez and Raikkonen follow.
The drivers' championship still wide open, but for the constructors Red Bull enjoy a healthy lead.
Many of the teams are running new engines this race - so there's plenty to check before running them.
Not too much happening at the moment - but the fans seem happy to soak up the rays and the atmosphere - it's 27C already this morning in Italy, and the setting is glorious.
That should mean we're getting near to the serious action - who can match McLaren's pace?
Every driver has now completed an installation run.
And despite the rumours on Lewis Hamilton's future - McLaren insist it's business as usual.
Ferrari not the fastest team so far this weekend however - McLaren dominated FP2 yesterday.
Italy, of course, is Ferrari's 'home' GP - so look out for some big names associated with the Scuderia. The cameras pan to Jean Alesi in the Ferrari garage, while Luca de Montezemolo, Ferrari chief, is doing the rounds as well.
Di Resta one of a number of drivers straight out on to the track. In case this is your first free practice session, don't expect any times to be set yet - this is the installation lap - when the drivers test everything is working before the serious stuff begins.
A bit of housekeeping - Paul di Resta had to change his gearbox and that costs him a five-place penalty for qualifying.
GO! One hour on the clock....
And here's how the standings are looking.
The weather is good in Italy today - that means that we're looking at three dry sessions in a row - which has not always been the case during these warm-ups on race weekends.
Hello and welcome to coverage of FP3 from the Italian Grand Prix at Monza.