The defending five-time world champion clocked an exceptional best lap in one minute and 16.568 seconds to outpace the young Monegasque by half a second in a session interrupted by two red flagged stoppages.
The first of these came when Hamilton's Mercedes team-mate and world championship leader Valtteri Bottas spun into a gravel trap after 20 minutes and the second in the 56th minute when George Russell hit the barriers in his Williams at Turn Four.
The drivers were unhurt, Bottas returning to the pits and later resuming to take third place ahead of four-time champion Sebastian Vettel in the second Ferrari. The Finn had been quickest in both Friday sessions.
In testing conditions, the American Haas team took fifth and sixth places with impressive efforts from Romain Grosjean and Kevin Magnussen, enabling them to beat the Red Bull of Max Verstappen, who was seventh.
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