A late lap from Sainz saw the Ferrari driver shoot to the top of the timesheets, bumping Alonso’s Aston Martin to second.
The pair both finished clear of Red Bull Racing’s Max Verstappen, who was third fastest, 0.238s away from the ultimate pace after the final practice hour.
It was a slow start to the session, Lewis Hamilton finally breaking the silence after seven minutes.
The seven-time champion had a set of red-walled tyres bolted onto his Mercedes, and spewed sparks as it bottomed heavily.
He recorded a 1:32.733s at a time when he was the only car on the track, circling back to the pit lane.
Sainz was the next man to head out, followed soon after by Ferrari team-mate Charles Leclerc.
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Others joined them, the session finally beginning with some 15 minutes having elapsed.
Times soon tumbled with Lance Stroll rising to the top of the timing screen, only to be beaten by both Ferraris and George Russell in the second Mercedes.
Russell’s best after 25 minutes was 1:31.821s, which compared favourably to the opening practice hour on Thursday, which saw Daniel Ricciardo log a fastest time of 1:32.869s.
Both were set on the soft compound tyres, in similar conditions – though Free Practice 3 began an hour later than Free Practice 1 had yesterday.
At Red Bull Racing, Sergio Perez reported harsh upshifts as he sat just 15th as the session approached its midway point, 1.8s away from the outright pace on the hard compound rubber.
The pace moved on to a 1:31.582s courtesy of Alonso, a 0.239s improvement over what Russell logged earlier.
Changes were made at McLaren for Lando Norris as team-mate Oscar Piastri sat towards the foot of the timing sheet, his best a 1:33.273s.
Up the order, there were seven different teams represented in the top 10 positions with 20 minutes to run, including the Haas of Kevin Magnussen in seventh.
With a little over 16 minutes remaining, Perez offered the first glimpse of what the Red Bull Racing was truly capable of.
On the soft tyres, he clocked a 1:31.248s on his first qualifying simulation lap to sit 0.334s clear of Alonso.
Verstappen went faster still with a 1:31.062s, the better part of two-tenths better than his team-mate despite losing time in the final sector of the lap.
Both Stroll and Alonso soon improved for Aston Martin, the former to fourth and the latter returning to the top of the timesheets by 0.097s.
McLaren too was having its drivers push on as Norris, with his suspension sufficiently tweaked, rising to third and Piastri fourth, 0.09s between the pair.
A late lap from Sainz saw the Spaniard leap to the top of the timing screens with five minutes to run in the session.
His 1:30.824s was 0.14s quicker than Alonso, who’d been 0.1s quicker than Verstappen.
With the field having completed its soft tyre run, Sainz’s time stood to the chequered flag, Alonso second fastest from Verstappen with Leclerc fourth and Norris fifth.
Piastri ended the session seventh best, with Daniel Ricciardo 11th in the first of the two RBs.
Qualifying follows, with drivers back from track from 19:00 local time (03:00 AEDT).
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