A mighty lap from the Dutchman to take top spot off Alonso by a whopping six-tenths.
Alonso sets an impressive 1m14.611s to go fastest - 0.2s ahead of his teammate.
Ocon, Bottas and Russell complete the top five.
It's a 1m15.482s for Russell - less than a tenth ahead of Verstappen. Hamilton completes the top three.
Norris, Magnussen and Albon - Williams are still preparing his car - in the pit lane.
Here we go for another 60 minutes of running in Monaco.
Sebastian Vettel has made his first paddock appearance of the year - his first since retiring.
He told the German press: "I've missed it, but I'm not here at Formula 1 because I missed it. I'm here at the invitation of Stefano (Domenicali)." And: "I'm doing very well, the time with the family is very nice, but it flies by."
Sainz fastest ahead of Alonso and Hamilton.
Red flag.
Our first on-track battle of the weekend?
— Formula 1 (@F1) May 26, 2023
Lewis and Max battle it out for the same part of the track #MonacoGP#F1pic.twitter.com/gsmRWGu43X
Sainz, Alonso, Hamilton, Perez, Leclerc, Verstappen, Norris, Ocon, Stroll and Albon.
The Ferrari driver is now on top - 0.058s ahead of Alonso.
Alonso pips Hamilton to top spot with a 1m13.907s, 0.1s ahead of Hamilton.
With over 17 minutes of the session to go, FP1 is back underway after the red flag.
Nico Hülkenberg brings out the Red Flag in Monaco! pic.twitter.com/3XYSJU8aLm
— Sky Sports F1 (@SkySportsF1) May 26, 2023
Hulkenberg didn't brush the barriers - he clattered them!
Replays show him losing the rear at the Nouvelle Chicane.
Red flag.
Looks like he's brushed the barriers.
"I can't really get my tyres working. I don't have the confidence. Like i'm just losing the rear exit of turn 3 and turn 4. The thing's just sliding."
"It's a little bit better on the rear, but yeah it's still doing al the same things on all the bumps it's not really good."
Hamilton, Sainz, Norris, Verstappen, Leclerc, Alonso, Albon, Perez, Tsunoda and Piastri.
It's a 1m14.035s for Hamilton - he's 0.2s clear of Sainz now.