Lewis struck a 46-ball 62, innings that was studded with four sixes and six fours, while Ottley's 30-ball 41 was decorated with two sixes and an equal number of fours as T&T managed to cross the 150-run barrier after being put in to bat at the Feroze Shah Kotla Stadium.
MI bowlers did well in the middle part of the innings to pull things back after Lewis' knock threatened to take the game away from the former champions.
Trinidad got off to a poor start as they lost their opener and last match's hero Lendl Simmons for nought in just the second over of the innings.
Mumbai skipper Rohit Sharma's ploy to introduce Harbhajan Singh early into the attack paid dividends as the off-spinner flummoxed the big-hitting Simmons with a tossed-up delivery.
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Simmons was caught at deep mid-wicket by Dwayne Smith, who took a well-judged catch at the edge of the boundary.
Australian pacer Mitchell Johnson managed to extract some bounce off the slow Kotla track but was taken to task by Lewis whenever he erred in length.
Lewis clobbered Johnson for four fours and a six over third-man fielder in his opening spell of two overs.
Lewis was in ominous form as neither spin nor speed proved a deterrent for him and his attacking instinct served the team well. He welcomed back Harbhajan for his return spell, clobbering biggies that cleared the ropes by some distance.