Karthik's 39-ball knock that included three fours and a six and his two significant partnerships with Sachin Tendulkar (24 in 27 balls) and Ambati Rayydu (22 in 39 balls) propped up the MI innings after they made a shocking start after being put to bat on a seam-friendly track.
MI lost two wickets in the second over, R Vinay Kumar accounting for both in a superb first spell with Zaheer Khan at the other end, before Tendulkar, who hit four fours, and Karthik stabilised the innings a bit with a stand of 39.
Tendulkar's dismissal at 41 in the ninth over saw Karthik and Ambati Rayudu joining hands to resurrect the stuttering innings with a stand of 44 in 33 balls.
But the dismissal of both these batsmen as well as Dwayne Smith in the space of 11 balls jolted the innings and left MI struggling at 100 for six.
Finally it was left to Harbhajan Singh (20 not out off 12 balls with two sixes) and the fit-again Kieron Pollard (21 not out off 13 balls with three fours) to hit some lusty blows to boost the score with an unbroken stand of 41 in 25 balls.
Pollard carted Vinay Kumar for successive sixes in the last over in which the medium pacer gave away 18 runs. Vinay Kumar ended up with two for 34 while Harshal Patel and Muttiah Muralitharan secured identical two for 24.
Mumbai Indians struggled to score runs on a wicket affording bounce and seam movement in plenty.
Both Zaheer and Vinay Kumar were impressive and the latter struck two big blows to send back left-handed opener James Franklin (1) and in-form number three Rohit Sharma (0) off the second and fifth balls of his first over, and the innings' second.