After Sri Lanka chased 430 to win in Rawalpindi, England's bowlers were on course to bounce back in Lahore on Sunday when they reduced Pakistan to 9-154 in pursuit of 242 – but Shadab had other ideas.
With just number 11 Haris Rauf for company, allrounder Shadab showed the urgency lacking from his top-order colleagues to register a maiden international hundred and get Pakistan home with 17 balls to spare.
Shadab marshalled the strike brilliantly in his 113 not out off 96 deliveries, allowing Haris to face just 19 balls from which he scored a crucial unbeaten seven, and it was England who blinked first.
They posted what seemed a below-par total of 241 all out, bolstered by Ben Duckett's majestic 105 off 111 balls, but after making early inroads into Pakistan's top-order, England's bowlers put on the collective squeeze with Jamie Overton collecting 3-43.
He had bowled out by the time Shadab and Rauf came together to inspire Pakistan and leave England facing a third successive defeat in the tournament, all but ending their bid to reach Saturday's showpiece at the Gaddafi Stadium.
Shadab brought up his fifty by belting England debutant Henry Crocombe for the first of five sixes while shielding Rauf continually from facing strike.
It looked to be in vain when he was yorked on 97 by Gus Atkinson and given lbw, prompting Shadab to throw his bat to the ground in disgust, but he was saved when a seemingly forlorn review showed up a thin inside edge.
Shadab went on to reach a magnificent 92-ball hundred courtesy of back-to-back fours off Atkinson.
That brought the target down to single figures and, when Shadab cracked Atkinson down the ground for six, the writing was on the wall, as he went to the highest score by a No.8 batter before Rauf sealed a scarcely-believable win by thumping a four down the ground off Crocombe.
England started the day by slipping to 3-19 for three when Harry Brook was run out for one, three days after his record-breaking double hundred, a slip when coming back for a second proving his undoing following Razaullah's bullet throw from the deep.
Duckett brought up his second ODI ton in Lahore, having made 165 against Australia at the Gaddafi Stadium in last year's Champions Trophy, before top-edging a sweep to Salman Agha when Arafat Minhas was introduced.
Atkinson belted three sixes in a breezy unbeaten 36.
