The Chiefs overcame a 17-point halftime deficit and capitalised on a controversial penalty try and yellow card to upset the Stormers 30-23 in their Super rugby clash in Hamilton.
The South Africans led 20-3 after dominating the first half but the Chiefs scored 17 unanswered points at the beginning of the second, sparked by the penalty try.
The Australian referee Stuart Dickinson awrded it when Richard Kahui was pushed in the back by Gio Aplon while diving for the bouncing ball in the in-goal area.
Aplon was also sinbinned and the Chiefs added a second try by Sitiveni Sivivatu and Stephen Donald slotted a penalty to tie the score at 20-20, while the Stormers winger was off the field.
The Stormers first-five Grant slotted his third penalty before Chiefs flanker Liam Messam scored to give the home side the lead which Donald extended with his third penalty.