Hamilton and Vettel won twice, five races into an exciting Formula 1 season, and the German leads ahead of Brit by six points.
In somewhat cloudy and warm conditions, Hamilton was .200 of a second clear of Vettel.
Red Bull driver Max Verstappen was third quickest, followed by Mercedes driver Valtteri Bottas, the other race winner this season.
Making his return to F1 as a one-off replacement for McLaren’s Fernarndo Alonso — who is skipping Monaco to compete in the Indianapolis 500 — veteran Jenson Button was 14th.
The second Red Bull speedy Daniel Ricciardo, who was the last man in the 1m13s brace. The Australian made fifth fastest ahead of Toro Rosso duo Daniil Kvyat and Carlos Sainz Jr., who spent the session running on Super Soft tyres, but still managed to place sixth and ninth respectively.
Kimi Räikkönen had a low-key start to the weekend in seventh position Sergio Pérez (P8), who grabbed a podium last year, and Esteban Ocon (P10), who has never competed in the Principality, put their Force Indias inside the top 10.
Lance Stroll was the other driver with no previous Monaco experience and wound up a second down on team-mate Felipe Massa, who was 11th quickest.