The duo had just nine caps between them at the start of the day but looked like seasoned campaigners as they despatched the home attack to all parts of Karachi’s National Stadium.
Brook blazed five sixes and eight fours as he made 81 not out in just 35 balls, while Duckett’s unbeaten 70 off 42 was littered with the instinctive sweeps he has built his reputation around.
Having come together in the ninth over the fourth-wicket pair smacked 139 without response to put their side in pole position.
Debutant Will Jacks settled immediately to life at the top of the England order after being picked ahead of Alex Hales, getting off the mark first ball and dragging Mohammad Hasnain to the mid-wicket boundary before the first over was out.
Phil Salt departed early, but Jacks dumped another over mid-wicket, muscled a drive straight past the bowler and thrashed Shahnawaz Dahani over cover.
His striking took England to 57 for one at the end of the powerplay, but leg-spinner Usman Qadir picked up a quick double when Dawid Malan (14) and Jacks (40 off 22) both picked out the sweeper.
Duckett had already helped himself to three boundaries when Brook joined him at 82 for three and they dovetailed brilliantly. Brook crashed Qadir for two sixes in four balls, middling the second even as the bowler tried to hang it outside off stump.
While he relied on power and timing, Duckett leaned into his reputation as an incorrigible sweeper and collected a series of fours that were both predictable but impossible to prevent.
Brook won the race to 50 with one big over off Dahani, threading two precise shots behind point either side of another big blow over mid-wicket. He had taken just 24 balls to reach the landmark, six faster than Duckett, who joined him a few moments later.
They scored 43 off the last three overs in increasingly confident manner, putting the finishing touches to a memorable partnership.