Verstappen closed out the opening 20 minutes of the hour-long FP1 session with a 1m22.657s, bettering his previous effort of 1m23.027s on the hard compound of tyres.
His Red Bull team-mate Sergio Perez had got to within three tenths to slot into the runner-up spot in the session but was beaten by Sainz's 1m22.703s just after the opening half-hour elapsed.
The alternative tyre allocation for the Monza weekend, following on from the first trial in Hungary, restricted drivers to the number of tyres they could burn through during the session as the customary 13 sets per car was reduced to 11.
This prompted a variety of approaches from each team, as some elected to save tyres in the opening part of the session to potentially keep an extra set free in qualifying.
Verstappen and Perez elected to stick with a set of hard tyres for the opening part of the session and traded times at the top with the Ferraris, Mercedes and Oscar Piastri on softs.
Sainz was first into the 1m23s on his hard compound of tyre, but Verstappen then found half a second on the Ferrari driver. During that opening 15 minutes, Piastri grabbed a 1m23.446s but this was again beaten by Sainz as the Spaniard got into an early groove in front of his team's home crowd.
Verstappen then pitched up with a 1m23.027s and then chipped almost 0.4s off his time during his next flying lap to extend his advantage over team-mate Perez. The two were then split by Charles Leclerc, the next driver to dip below the 1m23s barrier. Perez improved to reclaim second in the session but was then eclipsed by Sainz's time.
Source: Autosport