Bloemfontein: England made their second-highest one-day international total to beat South Africa by 39 runs on the Duckworth-Lewis method in Bloemfontein on Wednesday.
Jos Buttler's 73-ball century helped the tourists to 399-9, with Ben Stokes, Alex Hales and Joe Root passing 50.
Quinton de Kock's unbeaten 138 off 96 balls kept the Proteas in contention.
But the loss of wickets at key times halted the home side's momentum before rain ended the game with them on 250-5 to give England a 1-0 series lead.
The second ODI in the five-match series is in Port Elizabeth on Saturday.
The batting effort of captain Eoin Morgan's men represents a continuation of the dramatic improvement in England's limited-overs cricket since their woeful World Cup of 2015, when they were eliminated in the first round.
Since then, they have won a one-day series against World Cup runners-up New Zealand and Pakistan, as well as being narrowly beaten 3-2 by world champions Australia.
They have also won five consecutive Twenty20 internationals since the World Cup in an overall run of seven successive T20 wins.
Roy's opening partner Alex Hales then came to the fore, adding 62 runs with Joe Root to take England to 130 before he too was caught at cover, by AB de Villiers off the bowling of Marchant de Lange for 57.
Buttler then came to the wicket, where he would remain for 17 entertaining overs of clean hitting that propelled the tourists towards their huge total.
Buttler did not feature in the victorious Test series against South Africa, but he demonstrated why he is so valuable to the one-day side by hitting 11 fours and five sixes in an innings of 105 from 76 balls.
Remarkably, the 25-year-old Lancashire player's hundred was the slowest of his four centuries in ODIs, with his quickest being the 46-ball ton he scored against Pakistan in Dubai last year.
England - 399 for 9 (Buttler 107, Hales 57, Stokes 57, Root 52)
South Africa - 250 for 5 (de Kock 138*, du Plessis 55, Moeen 3-43)
Result- England won by 39 runs (DLS method)
MOM - Quinton de Kock (South Africa)