Hosts England made 311 for eight after being sent into bat by Du Plessis before bowling out the South Africans for 207 to start their campaign with an emphatic 104-run victory at The Oval.
"Today England were better in all three facets of the game," Du Plessis told reporters.
"But it’s a league comp, as disappointed as we were with our performance you have to learn from it and put every single doubt to bed as you have to play cricket again."
South Africa bowled and fielded tightly, restricting England's usually free-scoring batsmen to only three sixes in the innings and a modest 76 runs from the last 10 overs.
Although Jason Roy, Joe Root, Eoin Morgan and Ben Stokes made fifties, none were able to go on to three figures.
"I thought we did a good job with the ball," Du Plessis said. "England bat longer than anyone else so if you can stop them you are doing a great job. As few runs as they got in the last 15 overs – but it was a big score."
South Africa's reply was hampered by an injury to Hashim Amla who was forced to retire hurt after being struck on the helmet by a short ball from fast bowler Jofra Archer.
"The flow of the innings was affected with Hashim coming off, chasing a big score you need a good start, settled partnerships," Du Plessis said.
"Pitch was good enough, Archer bowled well with the new ball, it will take time for international players to get used to his action and that’s why he's such an X-factor bowler – he broke through our opening three."
Du Plessis was pleased with his tactic of opening the bowling with leg-spinner Imran Tahir who dismissed Jonny Bairstow with the second ball of the match to give South Africa a flying start.
"It was something we thought about a year ago," Du Plessis said. "It's something different. My gut said Imran had a big chance of getting a wicket – he's been practising with the new ball for a couple of weeks."
South Africa's next match is against Bangladesh at the same venue on Sunday.
(Reporting by Ed Osmond, editing by Toby Davis)
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