The Tigers expanded their lead to six games over Cleveland in the American League Central and reduced their magic number for clinching the division to 10.
Detroit built a 6-1 lead through five innings on the way to its third straight win.
Fielder's home run, his 24th of the season, snapped a 1-1 tie and Detroit added three runs in the fifth, one of them coming home on a Fielder single. The other two runs in the fifth scored on a base hit from Torii Hunter.
Justin Verlander (13-11) worked 6 2/3 innings to get the win, giving up three runs on nine hits with seven strikeouts. It was his first victory since Aug. 6, having gone 0-3 in six starts since then.
Bruce Chen (7-3) served up five runs on seven hits in 4 1/3 to take the loss. The Royals outhit the Tigers 10-7.