Magic hat-trick from Mackey is knockout blow for Racers
The 29-year-old needed just 15 seconds to show he had lost none of his predatory instincts.
Kai Williams’ powerful low drive was saved by keeper Eddie Caviezel-Cox and Mackey was on hand to stab in the rebound.
United’s strong start continued with four corners in quick succession before the injury-hit Racers, missing three centre-backs, rallied to force four of their own.
However, just as it looked as though they had gained a foothold in the match, Mackey doubled his and United’s tally, converting a square ball from another former Leamington player in Mykel Beckley.
Caviezel-Cox pulled off two world-class saves to deny the visitors a third, while the lively Jamie Smith almost pulled one back for Racing Club when he beat Richard Morris only to see Robert Prinzel clear off the line.
Smith then turned Prinzel inside out before shooting just wide.
On the stroke of half-time Mackey effectively killed off the tie with a well-timed header at the far post from a pinpoint Josh O’Grady cross.
Racing Club narrowed the deficit immediately after the break when Harry Jervis was brought down and Smith stepped up to send Morris the wrong way from the spot.
Racers enjoyed the better of the possession as the half wore on but never looked like getting themselves back in the game.
Racing Club are back in league action on Saturday when they travel to Nuneaton Griff.