Pakistan scored 277 for 7 in their 50 overs with Misbahul Haq hitting an unbeaten 83 and Younis Khan scoring 72 before restricting the home team to 266-9.
"That was a big game and the boys all knew how big that game was," said Afridi, praising Misbah and Younis for "very mature cricket".
Afridi, who took four wickets for just 34 runs in his 10-over spell to earn the man-of-the-match award, said the pitch offered "beautiful conditions" for both pace bowlers and spinners
He added that his team would put a sloppy fielding performance behind them.
"The boys were a little bit relaxed but we should tell them this is cricket, you don't, especially on a stage like this, relax. But I know that they will definitely do better in the upcoming games," Afridi said.
"I think it's a great message for other teams that we will come harder and harder game by game."
A disappointed Sri Lanka skipper Kumar Sangakarra praised his bowlers for keeping the Pakistan score down but said losing a clutch of wickets at the wrong time hurt them.
"Any defeat is tough but (there are) quite a few positives to take. I think we bowled pretty well on a flat deck," said the skipper, who struck 49.
"I think we made it a bit difficult for ourselves by losing wickets but we got a good start. We needed a couple of blokes to just kick on from there and keep batting for a longer time. Unfortunately Pakistan played very well," he added.
"I thought we dragged it back really well to keep them to 270. We chased pretty well but left a bit too much for ourselves to do at the end," he said.
COMMENTS (12)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ