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.
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