After showing his speed, Leclerc was 20 minutes away from completing a trouble-free session when his car came to a stop on a straightaway. His fuming Ferrari was loaded on to a flatbed and hauled away.
Leclerc's 138 laps were second to Williams rookie George Russell, who led with 140 laps on the day.
His stoppage was nothing compared to the scare that Vettel had a day earlier, when he crashed into a barrier and needed to be checked out by medical staff.
When asked if he had got any pointers from Vettel, Leclerc said that they their relation was far from student-teacher.
"At the end we need to work together to try and improve the car, but he also wants to beat me on the track as much as I want to beat him," Leclerc said. "So it's not like I'm going to him to try and have some tips, but I can definitely learn from him (by) looking at his data, looking at the way he works with the team. So,yeah, that has been very, very helpful."
Vettel will be back behind the wheel today, the last chance for teams to test their cars before the season-opening Australian Grand Prix on March 17.
Defending champion Hamilton had another mundane session, clocking the 10th-fastest time while sharing the wheel with Mercedes teammate Valtteri Bottas.
Bottas said Ferrari seemed to be a step ahead of Mercedes, who had won the title five years in a row.
"They still seem strong, (but) as always from testing to Melbourne a lot can change," said Bottas, who had the slowest time of the 13 drivers. "We need to unlock more from the new package to be able to compete with them properly."
Pierre Gasly ran his Red Bull off the track for a second time this pre-season. The Frenchman, who was promoted from feeder team Toro Rosso, lost control on curve Nine and hit the protective barrier.
Another red flag came out when Lance Stroll's Racing Point car stopped in a straight.
Along with Russell's long run, there were promising efforts from two more F1 newcomers. Toro Rosso's Alexander Albon clocked the second fastest time of the session, followed by Lando Norris (McLaren).
- AP