England 373 for 3 (Buttler 110*, Roy 87, Morgan 71*, Bairstow 51) beat Pakistan 361 for 7 (Fakhar 138, Babar 51, Asif 51) by 12 runs
Jos Buttler is on top of the world, and the biggest cricket show on earth hasn't even started yet.
The prelude was a corker though, with Pakistan falling just shy of a giant target of 374 to make England work for their 12-run victory and 1-0 series lead.
With a newborn daughter, Buttler showed no signs of sleep deprivation, but rather batted with the unbounded freedom that elite athletes speak of discovering when they return to action after becoming parents - the realisation that there is so much more to life than sport.
Buttler has been here before, his 100 off 50 balls en route to an unbeaten 110 off 55 the second-fastest England ODI hundred behind his own record, also achieved against Pakistan in 2015, long before the arrival of little Georgia Rose last month. But while his latest knock was vintage Buttler, it also looked to the future and he cradled his bat like a baby in celebration.
Buttler's remarkable ton capped a strong England batting performance in which all bar one man passed 50. He hit nine sixes and shared an unbroken 162-run partnership with Eoin Morgan, who reached 71 not out off 48, the pair building on a gritty innings from Jason Roy on his return from injury to steer England to 373 for 3.
The target required Pakistan's highest successful run chase and they gave it a good shot on a batting-friendly pitch at the Ageas Bowl, thanks largely to Fakhar Zaman's cool-headed 138, Pakistan's highest individual one-day score against England. By the time Fakhar was out, caught behind by Buttler while fishing at a wide one from Chris Woakes but only confirmed on England's review, Pakistan had reached 227 for 2 off 33 overs.
His dismissal swung the momentum back in England's favour but Pakistan - through Babar Azam and Asif Ali - made it a thrilling contest.
Having not played any cricket since April 23 when he suffered back spasms while playing for Surrey in the Royal London Cup, Roy returned at the expense of Hampshire's James Vince, who sat on the sidelines in an England shirt on the eve of his county's one-day semi-final against Lancashire at the same ground.
Roy, who had been battling injury since February's tour of the Caribbean, resumed the top-order partnership so crucial to England's recent success with Jonny Bairstow in an encouraging sign for home fans ahead of the World Cup. Bairstow brought up his 50 off 44 and could be forgiven for thinking he was not out on the next delivery from Shaheen Afridi, so understated was the celebration from Fakhar, who juggled the catch at deep backward square but managed to hold on and avoid sliding into the boundary rope.
Understandably, Roy took some time to look comfortable, but he blew the cobwebs away with some glorious shots, including a sumptuous six driven straight down the ground off Faheem Ashraf and a second off the same bowler which sailed impossibly high over extra cover before dropping into the stands, much to the surprise of the ducking spectators.
There were some scratchy moments but Roy retained more than a hint of his trademark aggression and batted with a palpable determination to re-establish his touch. His frustration over being dismissed as his century beckoned - to an easy catch by Imad Wasim off Hasan Ali for 87 - told just what a mission it had been as Roy thumped his bat on the ground and walked off with his hand covering his eyes. Roy had been 83 not out before a short rain delay and he added just four more runs before his dismissal.
Yasir Shah, in the Pakistan side for the ill Mohammad Amir, struggled to find his rhythm but his captain's perseverance eventually paid off, snaring the wicket of Joe Root after a neat cameo that yielded 40 runs off 54 balls before he was caught sharply by Haris Sohail diving to his left. Yasir was yanked from the attack at the end of the over - his sixth - after conceding 54 runs for his solitary wicket.
Morgan and Buttler brought up England's 300 with plenty of deliveries left to really start swinging, making the last five overs absolute carnage from a Pakistan perspective, yielding 74 runs.
The chase started brightly, the tourists reaching 92 before Moeen Ali's caught-and-bowled accounted for Imam-ul-Haq, whose 35 came in a supporting role to Fakhar, who had already reached his 50 off 39 balls. Fakhar brought up his century off 84 and he continued to frustrate England's bowlers until his rather anti-climactic dismissal.
Babar Azam followed in the next over to a regulation caught-and-bowled by Adil Rashid, who should also have had Haris Sohail caught by David Willey, who dropped the catch running in from long-on. It was Liam Plunkett who accounted for Haris upon returning to the attack in place of Rashid, with Bairstow holding a sitter at deep point.
Pakistan needed 98 off the final 10 overs and, with six wickets in hand and the Buttler-Morgan run fest fresh in their minds, Asif and Sarfaraz Ahmed attacked accordingly. Asif raced to a half-century before Willey - who bowled wonderfully at the death after rotating into the side for Jofra Archer - had him caught in the deep by Ben Stokes two balls later to remove a clear danger.
Buttler was crucial again taking a high-pressure catch off Willey to dismiss Imad Wasim and Plunkett chimed in with the wicket of Faheem to leave Pakistan needing 19 runs off the last over, bowled by Woakes, who held his nerve, restricting them to just six, including one wide.