Leclerc, who this week agreed a contract extension with Ferrari, won his home race in 2024 and has claimed pole position three times in the last five years, but he was out-qualified and out-raced by a revived Hamilton, who finished second, in Montreal two weeks ago.
In Canada, Mercedes' precocious championship leader reeled off a fourth consecutive win, including his maiden F1 triumph, to open a 43-point gap ahead of team-mate George Russell.
But, Antonelli admitted, he had enjoyed some luck and recognised too that Ferrari's more nimble car and power unit were likely to be better suited to the classic Mediterranean street circuit where Mercedes have often struggled.
The Silver Arrows last won on the unforgiving barrier-lined streets of the principality in 2019 when Hamilton was on his way to his sixth title. It was his third Monaco win, the most by any current driver, and he is relishing a return to one of his favourite circuits.
"It's the one track we go to where power is not king," he said. "It's definitely about the car performance and our car could be really strong there."
Antonelli, the boy prodigy mentored by Hamilton in his final days at Mercedes before moving to Ferrari last year, admitted: "I think Ferrari is the team to beat in Monaco. It's going to be very interesting to see how we do there."
Mercedes team boss Toto Wolff has played down their prospects of continued success.
"Monaco is different to all the other circuits and a car that has been competitive elsewhere is not guaranteed anything here," he said.
"The margins are small, the consequences are high.”
Last year, Russell finished 11th and Antonelli 18th, a result that Mercedes will seek to avoid repeating as they defend their championship lead after five straight wins.
Qualifying the key
For Ferrari, it represents a chance to claim a first win since Carlos Sainz triumphed in Mexico in 2024 and to confirm their renaissance with a car well-conceived for the new hybrid era.
As always, Saturday's qualifying session is likely to be a decisive factor, but many observers believe the "yo-yo" characteristic of this year's racing – created by the use of a new overtake mode can create more possible overtaking moves in the race.
It gives a driver, who is within a second of another car, an extra 0.5 MJ of electrical energy from the power unit, which has a 50-50 division of traditional engine and battery.
Last year's race saw world champion Lando Norris dominate with a classic hat-trick of pole position, victory and fastest lap for McLaren, the most successful team in Monaco with 16 wins since their F1 debut there in 1966.
The team hope to celebrate their 60th anniversary and 1000th race start by thwarting Ferrari and Mercedes, with Red Bull likely also to be in contention through not only Max Verstappen, a two-time Monaco winner, but also the talented French driver Isack Hadjar.
He was fifth on the grid last year for Racing Bulls and finished sixth in the race in what was an impressive rookie season.
The race has been shifted to a later date in the calendar, after traditionally occupying the Ascension Day weekend, and is the first of a run of six races in eight weeks, with the teams moving on immediately to Barcelona.
The weekend will see Cadillac make their maiden race appearance at the glamorous "jewel in the F1 crown", delighting their American audience as the sport continues to attract viewers in the United States.