South Africa had scored just 44 runs when Quinton de Kock was caught behind off Nuwan Pradeep's bowling in the 13th over and the fast-medium bowler nearly had his second wicket shortly after as du Plessis, on 8, skied a hook shot into the leg side.
South Africa now sit top of Group B ahead of Sunday's clash between India and Pakistan. While de Villiers now has a reputation that means even when he plays a pull too early and gifts a catch, as he did today, he will still steal headlines, de Kock has been South Africa's most successful run-scorer since the 2015 World Cup and his precocious talent will result in him being recognised for many a year to come.
AB de Villiers was reluctant to modify his strategy upfront as he persevered with Kagiso Rabada (1-46) and Wayne Parnell (0-54) for a full nine overs before he considered a bowling change.
The total was less than expected but more than enough as Sri Lanka was bowled out for 203 in the 42nd over, with Tahir taking 4-27.
South Africa have only failed to progress from the group stage twice in ICC Champions Trophy history, and they laid the foundations for yet more success with an impressive opening performance in London.
Three deliveries later, Chamara Kapugedera was foxed by a Tahir googly and was out leg before wicket without scoring.
Under the Code, Sri Lanka's players were fined 60 per cent of their match fee for being over time.
Tharanga also added that skipper Angelo Mathews, who almost missed out on returning to full fitness for the game against South Africa, will most likely be available for the next match against the defending champions.
145 - The second wicket partnership between Hashim Amla and Faf du Plessis is the highest by a South African pair against Sri Lanka in ODIs, beating the previous best of 144 by Amla and Kallis in 2012.
Despite losing Chris Morris for 20, JP Duminy smashed an unbeaten 38 to lead his side to 299/6 off their 50 overs.
"We got a couple of wickets to turn the momentum around a bit", de Villiers added.
Once he departed, also accounted for by Tahir, Sri Lanka never really had any chance to provide any kind of fight.
"We play against India next and we have to come back strongly, especially the batting".
Upul Tharanga suspended for two of the upcoming Champions Trophy match. Hashim is a great asset to have. Tahir went on to pick up four wickets and was named as the Player of the Match for his efforts. I'm working hard and I hope this form continues in the tournament.
"Imran was close to 10 out of 10".
He took on the responsibility South Africa needed against a Sri Lanka attack that was not entirely threatening but bookended the innings with fairly tight spells. Always at these tournaments, a big boy finds a banana skin, and is surprised by a Bangladesh or in this case, a rebuilding Sri Lankan outfit.