Red Bull's Verstappen, winner last year at Austin's Circuit of the Americas, leads McLaren's Norris by 52 points with six rounds remaining.
Spaniard Sainz lapped with a best time of one minute 33.602 seconds with Leclerc, on pole last year, 0.021 seconds slower.
Verstappen, who set his best lap earlier in the session, and Norris were 0.253 and 0.266 off the pace respectively.
Red Bull have made an adjustment to their car after a controversy erupted on Thursday over a ride height device that triggered concern from rivals.
McLaren had to make changes to their rear wing after the previous race in Singapore, with the technical battle heating up as the title chase enters the final quarter of the season.
Austin hosts the fourth sprint weekend of the season, with eight points available on Saturday to the winner of the 100km race. Qualifying for that race followed later on Friday instead of the usual second practice.
Sainz told reporters earlier he expected Austin to be an acid test of Ferrari's upgrades, introduced at the team's home Italian Grand Prix at the end of August, and the immediate evidence was promising.
The two Ferraris were one-two at the half-way point on hard tyres, before Verstappen went faster as one of the first to switch to softs. The pair then returned to the top when the softs were bolted on.
McLaren's Oscar Piastri was fifth fastest with Mercedes' seven times world champion Lewis Hamilton sixth after an early spin in a car with significant upgrades.
Mercedes' George Russell, who also spun, was seventh on the timesheets with Haas's Kevin Magnussen eighth and ahead of Aston Martin's Fernando Alonso.
RB's Yuki Tsunoda rounded off the top 10, with teammate Liam Lawson 13th.
New Zealander Lawson has replaced Australian Daniel Ricciardo for the last six races but will have an inherited grid penalty in Austin.