Vettel was closely followed by Alexander Albon in the other Red Bull, with the energy drinks squad showing strong pace around the Circuit of the Americas.
Throughout the session many drivers complained that the circuit was very bumpy, with Lewis Hamilton questioning whether the track was safe.
Pierre Gasly was just under a second slower than Verstappen but nevertheless finished in fourth place, with the session results skewed slightly as some teams were testing tyres for Pirelli.
One such driver was Hamilton, who initially set a lap time good enough for P4 but saw his effort deleted due to track limits violations. He later recovered to finish in eighth place.
Next up came Romain Grosjean in sixth place with a much stronger showing from Haas after a disastrous lack of pace in Mexico. He was closely followed by Leclerc, with the Monegasque driver just 0.024s slower.
Leclerc was another driver to have his best time deleted however, as he had originally managed to split Verstappen and Vettel with a lap just six-hundredths slower than the Dutchman.
Carlos Sainz will have hoped for a better session, as the Spaniard 'destroyed' his tyres after a spin during the session. Some teams struggled to get heat into the tyres due to the colder than usual weather that they are facing in Austin.
Bottas was another driver taking part in the tyre test and finished down in seventeenth place, with Lando Norris just behind the Finn.
Nicholas Latifi took over from George Russell in the Williams as the team evaluate their 2020 driver lineup. However, unfortunately for the Canadian his session was ended prematurely with a gearbox issue.
Bahrain International Circuit - Winter testing