The Surrey batsman sought medical attention in the early exchanges of England’s innings in the hosts’ rain-hit 44-run victory, struggling to balance and pick out the ball.
Play was halted for five minutes while Roy received on-field treatment, but the 26-year-old refused to abandon the crease, before producing a tenacious match-turning innings.
England restricted Pakistan to 260 for six from their full 50-over allocation in the first of the five-match series, before Roy wrestled control in the middle.
As England confirmed Roy’s problem as dizziness caused by heat, Joe Root and captain Eoin Morgan set about guiding the hosts home.
Linchpin Root struck a hassle-free 61 before succumbing to Azhar Ali’s easy run out from Morgan’s poor call, but England still coasted home.
Two quick-fire rain delays ended the match, with Morgan unbeaten on 33 and Ben Stokes on 15 not out, and England victorious with their total of 194-3 on the Duckworth-Lewis method.
Roy’s early complaints over apparent blurred vision in no way blighted his batting clarity.
He received five minutes’ treatment after a quick-fire 20 from 14 balls, appearing to complain of dizziness and vision issues.
The South Africa-born batsman soldiered on however, quickly easing past his light-headed spell.
Certainly it seemed an energy drink helped him recover his poise, after which his only concern was when Sarfraz Ahmed clumsily put him down on 24.
Opener Alex Hales proved England’s sole disappointment with the bat, mustering just seven before Umar Gul had him caught at slip.
Solid, if unspectacular bowling had earlier teed up England’s chance for victory.
Adil Rashid claimed two scalps and Mark Wood completed a wicket-taking return after a year’s absence with ankle problems.
Neither Ali’s measured 82 nor Sarfraz’s 55 from 58 balls proved sufficient to put the tourists in the box seat.
Wood set England off by trapping Sharjeel Khan caught behind for 16.
The Pakistan opener let hubris dictate his decision to demand a review, only to be rightly denied a reprieve.
Part-time spinner Root delivered the second wicket, Mohammad Hafeez’s top edge gifting Hales the chance to redeem himself for the earlier dropped catch with Pakistan reduced to 52-2.
Babar Azam built a steady 40 before being dismissed leg before – in an incorrect call that could not be overturned as the tourists had already wasted their solitary review.
So Babar had no argument when Rashid’s appeal was granted, even though replays showed he edged the ball before it struck his pads.
Azhar reached his 50 in 84 deliveries, noticeably opting to step up the pace after that.
Moeen Ali held firm when Azhar skied one off Rashid however, with Pakistan’s skipper and innings linchpin removed with his side 178-4.
The day’s searing sun then gave way to the expected rain showers, and after a 20-minute hiatus Shoaib Malik was quickly dismissed by Liam Plunkett, for 17 runs.
Pakistan’s 224-5 became 226-6 shortly after, with Sarfraz’s relatively quick burst over at 55 in 58 balls.
Imad Wasim with 18 and Mohammad Nawaz’s 17 guided Pakistan to 260 from their full allocation.