Dave Donnelly reports from Richmond Park
Achille Campion's first St Patrick's Athletic goal kicked off an outstanding second-half performance as the Saints stunned Waterford.
Mick Leahy scored his first and second league goals for the Saints as Liam Buckley's decision to experiment with three at the back paid off after a shaky start.
The Blues drop out of the automatic European spots as a result of a performance that never really met the standards set by Alan Reynolds' side this season.
Buckley brought Leahy into the side alongside Lee Desmond and Kevin Toner, partly in an effort to dull the aerial prowess of Ismahil Akinade and Courtney Duffus up front for the Blues.
The trio largely kept the front pair quiet, and Waterford's direct tactics yielded little dividend, but it did leave the Athletic looking oddly blunt in the attack third in the first 45 minutes.
That all changed after the break as the Saints emerged with a fluency and desire that rattled their opponents, who have caused a few shocks themselves this year with a mix of power and pace.
Campion opened the scoring nine minutes after the break as he finished into an empty net following a flowing move that had been started and almost finished by the electric Darragh Markey.
The midfielder robbed Dessie Hutchinson on the edge of his own box before setting Ryan Brennan – a first-half substitute for Conor Clifford – free on the left.
He found Jake Keegan, who cut back onto his right foot before finding Markey in the box and his shot was parried by Matthew Connor into the path of the awaiting Campion.
The Athletic continued to pour forward and there was a sense of inevitabiity about the second as Leahy nodded home Markey's corner with the back of his head.
The Blues did belatedly force a chance of their own, Akinade curling just wide from the edge of the box, but they never looked like getting back in the game.
And so it was left to Leahy to have the last say, the defender getting his second double in Saints colours as he headed home Ian Bermingham's free kick.
St Patrick's Athletic: Brendan Clarke; Mick Leahy, Kevin Toner Lee Desmond; Simon Madden, Jamie Lennon, Conor Clifford (Ryan Brennan 25); Darragh Markey, Ian Bermingham; Jake Keegan (James Doona 85), Achille Campion (Ian Turner 90).
Subs not used: Barry Murphy (gk), Michael Barker, Conan Byrne, Dean Clarke.
Booked: Darragh Markey (41).
Waterford: Matthew Connor; Rory Feely (Paul Keegan 67), Kenny Browne, Dave Webster, Dylan Barnett (Noe Baba 89); Dessie Hutchinson, Bastien Hery, Gavan Holohan, Sander Puri (Carlos Wilson 71); Courtney Duffus, Ismahil Akinade.
Subs not used: Niall Corbet (gk), Darryl Walsh, Reinars Rode, Dean O'Halloran.
Booked: Bastien Héry (15), Kenny Browne (64).
Referee: Ben Connolly.
Attendance: 1,428 (official).
Extratime.ie Player of the Match: Mick Leahy (St Patrick's Athletic).