Williamson had made 67 off 44 balls before he was dismissed.
Earlier, Stoinis had scalped another in-form batsman Manish Pandey with a similar delivery.
In Tuesday's final too, Stoinis's variations will hold the key if DC are to contain MI's power-hitters like Kieron Pollard, Hardik and Krunal Pandya.
"My thought process was to change my grips and see what I was getting out of the wicket and change my tactics with wide yorkers and slower balls," said Stoinis after Sunday's match. "Bowling in this format is always hard. It is all about planning before you get to the ground, you have to get a good idea, you are speaking to your captain and coach, and different batsmen in your strategy."
MI vs DC final playing 11 prediction and head to head
Stoinis, who gave away just under nine and picked three wickets in Sunday's game against SRH, has had a decent outing with the ball this season.
He has now picked 12 wickets at 21.66, his best bowling average in his IPL career. This is apart from the 352 runs he has got with the bat for his side.
Stoinis feels the key is to build pressure with the ball. That comes with dot balls. Of the 166 deliveries he has bowled in this IPL, 47 have been dot balls. It has also helped the team's and his cause that DC's two strike bowlers Kagiso Rabada and Anrich Nortje feature among the top five with most dot deliveries this season.
"Important to stick to your strengths. In T20, you are not often taking wicket but also control the run rate and control the game, build some pressure with which u get some wickets," Stoinis added.
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