George Russell and Lewis Hamilton were next in their Mercedes, which have struggled to keep up with Ferrari and Red Bull this season. They were within 0.2 seconds of Leclerc.
The 27-year-old Sainz is third in the drivers’ standings, 38 points behind Leclerc and four below George Russell.
Verstappen's time of 1 minute, 31.94 seconds put him a narrow 0.09 ahead of Charles Leclerc and 0.58 clear of Carlos Sainz Jr on an encouraging day for Ferrari.
Carlos Sainz said that he's happy Austin's Circuit of the Americas is the venue for his first race with new team Renault.
Renault's managing director Cyril Abiteboul thanked Red Bull, Toro Rosso's parent company, for loaning Sainz.