LONDON: South Africa chose to bat and laboured to 142 for three before Bairstow and Alex Hales shared a superb unbroken second-wicket partnership of 98 to guide the hosts to their target with 5.3 overs to spare.
“I’m trying to progress all the time and I played a few shots I didn’t have a few months ago,” said man of the match Bairstow.
“I’m feeling good, I was really pleased. The lads bowled outstandingly well, to take wickets up front is what we wanted and then our spinners tied them down.”
David Willey bowled JJ Smuts with the first ball of the match and South Africa quickly subsided to 32 for three.
Captain AB de Villiers (65 not out) and Farhaan Behardien (64 not out) shared an unbroken stand of 110 but they never broke free from the shackles imposed by a disciplined England attack and the total always looked below-par.
England openers Jason Roy and Hales plundered 45 off the first four overs before Roy, playing a risky reverse sweep, was trapped lbw by Andile Phehlukwayo for 28.
But Hales (47 not out) and Bairstow looked completely untroubled by a toothless South Africa attack, hitting four sixes and nine fours between them to cruise to their target.
Both teams were playing their first matches since the Champions Trophy in which hosts England lost to Pakistan in the semi-finals and top-ranked South Africa failed to advance from the group stage.
“It is difficult to sum up the performance. The result doesn’t look good for us. We lost our way at the start, had to rebuild and we were 20-30 runs short in the end, which probably cost us,” De Villiers said.
The second game of the three-match series is in Taunton on Friday.