Rory McKenzie set Kilmarnock on their way to victory with a debut goal as Dundee United were knocked off the top of the Scottish Premier League.
Jon Daly squandered two good chances as United edged a first-half stalemate.
McKenzie pounced on a defensive error to open the scoring a minute after the break and Borja extended the lead from the penalty spot four minutes later.
Jude Winchester forced home a third after 56 minutes before Daly rolled in a reply with 11 minutes remaining.
This was a stunning result given the early season form of the two teams, but the home side deserved the win after a devastating 10-minute spell at the start of the second half and some schoolboy defending by the visitors.
Managers Kenny Shiels and Peter Houston had a difference of opinion ahead of kick-off, with the former suggesting the visitors' football was "direct and effective" but not the "most attractive" in the division.
Houston cited two 3-0 wins at Tannadice as reasons to believe the visitors might just be a force to be reckoned with in the race for second.
However, neither man could have foreseen what would happen in a compelling 90 minutes.
The first half was largely forgettable, with the clash of heads involving Gavin Gunning and Gary Harkins on 12 minutes a real concern for the 4,516 inside Rugby Park - Harkins was carried off.
Winchester celebrates after scoring Kilmarnock's third goal
Even then, the smart money would have been on United stepping up a gear to try to stay top of the table, but the visitors' defence was about to self-destruct.
Who knows what was going on inside Barry Douglas's head less than two minutes after the interval when he set off on a mazy dribble across his own six-yard box, only for Killie debutant Rory McKenzie to nick the ball off him and despatch it past Rado Cierzniak from two yards for probably the easiest goal he will ever score.
Two minutes later and the lead was doubled when Gary Mackay-Steven tripped Danny Racchi in the box.
Referee Crawford Allan pointed to the spot, and the Spaniard Borja Perez beat Cierzniak with ease.
Killie's third came from a 55th minute corner which was not cleared and substitute Jude Winchester wheeled to thrash the ball past the keeper from six yards.
Jon Daly's 50th SPL goal with 12 minutes left - his second of the season - gave United some hope, but with Harkins now stitched and watching from the directors' box, it proved to be Kilmarnock's day.
It means Killie have defeated all SPL clubs except St Johnstone since Kenny Shiels took over at Rugby Park.
Houston's players will face some very uncomfortable moments when they watch a replay of this match in the coming days.
The manager certainly felt it would "clarify the thinking" of anybody who thought United would simply have to turn up to win.
- 17 Letheren
- 02 Tesselaar
- 04 Fowler
- 05 O'Leary
- 06 Nelson
- 07 Racchi
- 08 Kelly
- 11 Harkins (Winchester - 15' Booked )
- 15 Perez (Johnson - 73' )
- 29 Pascali Booked
- 24 McKenzie (Gross - 81' )
- 53 Jaakkola
- 26 Pursehouse
- 10 Dayton
- 18 Johnson
- 28 Winchester
- 09 Gross
- 16 Boulding
- 01 Cierzniak
- 02 Dillon
- 03 Douglas
- 05 Gunning
- 12 Watson
- 06 Flood (Dow - 80' )
- 08 Rankin
- 10 Ryan (Gardyne - 53' )
- 11 Mackay-Steven Booked (Armstrong - 53' )
- 07 Russell
- 09 Daly
- 13 Banks
- 04 McLean
- 14 Millar
- 16 Armstrong
- 28 Robertson
- 15 Gardyne
- 18 Dow