Regardless of Verstappen taking five places for a new combustion engine, Leclerc set the pace at Monza by a quarter of a second to the delight of the home fans as Sainz, who starts at the back of the grid, clocked. third over Sergio Perez.
Ferrari had provisionally secured a 1-2 in Q3 as Sainz delivered 1m20.584s on his first run in Q3, aided by a purple run through sector one, to edge out Leclerc’s 1m20.770s effort.
Verstappen initially settled for third, despite his second sector prowess, as he was just under a tenth behind the second F1-75.
And then for the climax, it looked like Red Bull might provide one last answer as Verstappen once again drove fastest through the middle part of the circuit to pile on the pressure.
Although Leclerc had been a few hundredths slower than Sainz’s benchmark in S1 and had never stitched together a purple sector, his overall effort scored 1m20.161s.
That gave him bragging rights over Verstappen’s 1m20.306s, even before the RB18 takes sixth on the grid as Sainz ran to 1m20.429s.
With Perez down 10 places – the Mexican has been a league behind the top three as he 0.8 sec. down on sister Red Bull – and Lewis Hamilton another driver exceeding his permitted share limit, George Russell will start alongside Leclerc.
This was despite the Mercedes W13 again struggling to warm up its tyres.
Lando Norris, meanwhile, was seventh fastest over Daniel Ricciardo as Pierre Gasly did enough to pip Alpine’s Fernando Alonso – the two-time champion aborted his final run.
Gasly had rocketed into the top 10 shootout thanks to a 1m22.062s effort in Q2 to find 0.07s over Esteban Ocon, the Alpine rider missing out on 11th place after going slower in the first and second sectors compared to his previous lap.
Gasly escaped the attention of the stewards to be released alongside Lando Norris as part of a late storm to find track position as everyone exited the garages with 2m20s remaining.
Valtteri Bottas, who struggled on the brakes throughout practice, raced to 12th ahead of Formula E champion and Alex Albon replacement Nyck de Vries aboard the Williams.
De Vries, in his first ever F1 qualifying session, had complained that he had struggled on the brakes into the first harassment, but his final flying lap was shot later in the lap.
Approaching the second chicane, the Dutch driver locked up the rear to suffer a big snap which he eventually caught, but immediately knew his qualifying session was over.
Nevertheless, he put the FW44’s linear speed supremacy to good use to grab 14th place ahead of Zhou Guanyu’s Alfa Romeo and Yuki Tsunoda.
The AlphaTauri driver, who joins Hamilton and Sainz with a back of the grid penalty, did not take part in Q2.
It came after Verstappen became the first driver of the weekend to dip below 1m21s, having topped the 18-minute Q1 by 0.35s over Leclerc thanks to his 1m20.922s flyer.
But it was less rosy for Nicholas Latifi, whose struggles on the brakes eliminated him in the first part of qualifying, running just two hundredths slower than de Vries’ first time.
De Vries sat after an appendicitis-sidelined Albon and held on for a Q2 appearance despite his last fastest lap in Q1 behind to push his luck with track limits.
Sebastian Vettel, meanwhile, continued his run of Q1 exits in 17th, while Lance Stroll was only 18th in the second Aston Martin as Haas fell behind.
It was a messy session for both VF-22s as Kevin Magnussen had his times deleted twice for exceeding the track limits thanks to a series of offenses by the other Lesmo.
Mick Schumacher, hampered in practice by clutch problems, ran slowest after a significant front-right lock-up in the first chicane, which ran straight on.