A disappointing Waterford dropped to fourth in the Airtricity Premier Division as St Patrick's closed the gap between the pair to seven points with an emphatic win in Inchicore.
A long-odds brace of goals from Mick Leahy after Achille Campion's opener keeps the Saints' slim European hopes alive.
Waterford will have serious questions asked of them after a listless display here, with Shamrock Rovers overtaking them in third place.
In a way, this was a game from another era: both sides played two up top, exceedingly rare nowadays. However, Patrick's went with an uncharacteristic 3-5-2, while Waterford effectively played with no wingers, their usual narrow four in midfield.
A lively opening saw Patrick's on the front foot. Bastien Hery was robbed of possession by Jake Keegan, the American's ambitious lob clearing Matthew Connor's posts.
Courtney Duffus, who struggled to hold the ball up at times, nearly profited from a mix-up at the back on 19 minutes and Waterford grew into the game, Sander Puri twice coming close midway through the half.
Initially, he saw Brendan Clarke parry his drive after superb play by Hery; moments later, a left-footed Puri drive after Izzy Akinade set him up was again saved by Clarke.
Sloppy play from former Saint Kenny Browne gave the hosts an opening on 32 minutes. Campion profited and squared to Keegan, whose shot was wayward, though his balance made it difficult.
The half ended with Puri again advancing to strike at Clarke. He spilled the ball, which almost fell for Akinade, but Kevin Toner hoofed it clear.
Patrick's again started well after the interval, with local boy Ian Bermingham composing himself but clearing Connor's left-hand post.
Then Darragh Markey put Keegan in, but Connor got down to save.
Poor defending from Waterford resulted in the opening goal nine minutes after the restart. Markey was in loads of space in the box and, his effort parried by Connor, Campion had a tap-in to make it 1-0.
Akinade and Duffus combined well just before the hour-mark but the last-named dragged his shot badly wide. It was really lively now and, Connor stranded, Simon Madden's cross deflected narrowly over his bar.
Pat's made it 2-0 midway through the second half. Bermingham's corner was met by the impressive Leahy, who headed into the far corner.
Campion had a fine game and headed off the post with 17 minutes left, as the Blues looked bereft of ideas. However, Hery set up Duffus, who wriggled to find a little room, but his shot faded wide.
Pat's looked in control and sealed the victory with another Leahy header with seven minutes left, again from a superb Bermingham delivery from a corner.