Jarrod Bowen continued to enhance his reputation as one of the best players in the Championship with two goals as Hull hammered Preston 4-0 at the KCOM Stadium.
The 22-year-old broke the deadlock in the first half, taking his tally for the club to 50, and rounded off the scoring with his 14th of the season.
Alex Neil's side lost further ground on the automatic-promotion spots, slipping six points off second-placed Leeds after a second consecutive defeat.
It could have been so different for Preston had Sean Maguire's curling shot not cannoned back off the post in the fourth minute.
Paul Gallagher also had an effort deflected wide in a lively start from the visitors.
Hull responded and only desperate defending denied Grosicki from scoring, with a combination of Patrick Bauer and Declan Rudd deflecting the ball over the top for a corner.
City got their first goal in the 30th minute through their talisman, albeit in fortunate fashion.
Grosicki's cross was deflected back across goal by Joe Rafferty into the forward's path and he tapped in from close range for his landmark strike.
Not for the first time, Preston got in behind the Hull defence, with the locals screaming for offside, but Daniel Johnson sliced a shot wide from Billy Bodin's cross.
Rudd then kept Preston in the game with a string of fine saves before the interval.
The goalkeeper parried a Grosicki low shot following a fine run, tipped over a fizzing Josh Bowler drive, prevented a Bowen effort from squeezing underneath him and then kept out Magennis' acrobatic close-range backwards volley.
However, Rudd could do nothing to stop Hull from sealing the points with two goals in the first six minutes of the second half.
Magennis scored from the penalty spot after Ryan Ledson had taken down Bowler, before Grosicki converted Bowen's cut-back to complete a superb move.
Things could have got worse for Preston had Jackson Irvine not skied a rebound from close range after Rudd parried out Grosicki's low free-kick.
Preston almost set up a nervy final 20 minutes, but Bauer found the side-netting and Josh Harrop struck the post.
And Bowen had the last laugh in the 77th minute, breaking through and lashing home from a tight angle, and still had time to volley over as he eyed a hat-trick.