Leclerc had the ever-passionate fans on their feet by lapping Monza's 'Temple of Speed', celebrating its 100th anniversary, with a top time of one minute 22.410 seconds on soft tyres in the first session.
Sainz, who will start Sunday's race from the back of the grid after engine penalties, was 0.077 slower but fastest in the later stint with a lap of 1:21.664. Leclerc was third and McLaren's Lando Norris (22) fourth.
The Mercedes pair of George Russell (24) and seven times world champion Lewis Hamilton were third and fourth fastest respectively in session one and then fifth and seventh.
Hamilton, a five-time winner at Monza, will also be starting from the back on Sunday due to a raft of engine-related penalties.
Red Bull's runaway championship leader Max Verstappen (24) was fifth and second in the sessions.
The Dutch driver, 109 points clear of Leclerc with seven races remaining, has a five place penalty due to exceeding his engine allocation with the eventual grid turning into a mathematical conundrum.
Verstappen's Mexican teammate Sergio Perez (32) will drop 10 places while AlphaTauri's Yuki Tsunoda (22) is also due to go to the back of the grid for a combination of accumulated reprimands and penalties.
Alfa Romeo's Valtteri Bottas (33) has incurred a 15-place drop and Haas's Mick Schumacher (33) also faces a penalty after he stopped on track with a power unit problem.
The eventual shape of the back end of the grid will be decided by qualifying on Saturday.
STAND-IN DE VRIES
At the bottom end of the timesheets Dutch driver Nyck de Vries (27) stood in for Sebastian Vettel (35) at Aston Martin in the first session - the third team he has represented in practice this season after Mercedes and Williams.
Ferrari's Italian reserve Antonio Giovinazzi (28), dropped by Alfa Romeo at the end of last season, replaced Schumacher at Haas for the same session and was 18th.
The teams - a majority of them based in England - stood in the pit lane outside their garages to observe a minute's silence before the session started with the drivers wearing black armbands.
A photograph of the Queen, whose parents attended Formula One's first world championship grand prix at Silverstone in 1950, appeared on a big screen over the podium.
World championship leaders Red Bull said they had pared back social media and silenced the music usually playing in the garage while mechanics are working.
Teams also carried messages of respect on the cars for Britain's longest reigning monarch, who died aged 96 on Thursday.
Hamilton, who received an honour from the Queen after his first championship in 2008 and was knighted last December by the now King Charles III, posted a message on Instagram.
"She was a symbol of hope for so many and she served her country with dignity, dedication and kindness," he said. "She was truly like no other and I'm grateful to have lived during her time."
Formula One's Italian chief executive Stefano Domenicali spoke of the "huge respect for what she did, and what she’s leaving as a legacy."
