The 47-year-old summoned back-to-back centuries as he pounced on his opponent's evident nerves to stay on course for a title he may have made his own, but has not won since 2017.
Despite frequent hints at retirement and having fallen out of love with the game, O'Sullivan said he felt refreshed and focused and fired a warning to the rivals who want to take his crown.
O'Sullivan told the BBC: "I feel like I've got life in perspective - I spent enough years getting disappointed by the game, and I thought let's try not to get disappointed, and it's freed me up a bit.
"I'm still competitive. If I take up anything I want to be better, it's just natural and I'll never lose that, so the players can expect me to keep coming for more."
The ruthless nature of O'Sullivan's triumph over the Belgian former UK finalist underscored the extent to which he is eager to extend his dominance of this prestigious event.
After O'Sullivan opened with a break of 97, Brecel missed golden opportunities to win each of the next two, running out of position on the brown in the second then going in-off the blue in the next having potted an astounding green off the side cushion with the rest.
O'Sullivan swept up to go 3-0 in front and although Brecel responded with a century before the mid-session interval, a lack of concentration cost him in the fifth frame when a poor break-off shot let in O'Sullivan for a clearance to black of 134.
A missed red to the middle in the next let in O'Sullivan for a 104 before he completed a win which will have sent a clear message to his title rivals.
"You sense someone is a bit nervous and you just pounce on it," added O'Sullivan. "Whoever you are in any sport, when you play the top players they will sniff it out.
"You don't have to say you're under pressure, I smell it, and if I smell it I'm going to be on you like a rash."
Two-time champion John Higgins exited after a 6-3 defeat by Jack Lisowski, who claimed his first Masters victory at the fourth attempt.
Higgins, the 1999 and 2006 winner, made a scrappy start and fell 2-0 behind before reducing arrears with a 142 clearance - the highest break of the tournament so far.
The players traded frames before Lisowski's century break put him 4-2 up. Higgins had chances to cut his opponent's lead to one frame for a third time but Lisowski moved 5-2 ahead.
A 93 break in the ninth frame wrapped up the match for Lisowski, who will take on Iran's Hossein Vafaei in the quarter-finals.