The African minnows are on course for a second straight Super 12 place after achieving the feat on their T20 World Cup debut last year in the United Arab Emirates.
Namibia lost their openers inside three overs with returning fast bowler Dushmantha Chameera getting a wicket third ball when Michael van Lingen was caught at deep third-man.
Jan Nicol Loftie-Eaton fell to a brilliant diving catch by wicketkeeper Kusal Mendis after a 12-ball 20.
Stephan Baard and skipper Gerhard Erasmus attempted to rebuild with a stand of 41 but Sri Lanka’s bowlers kept chipping away with wickets.
Namibia slipped to 93-6 when spinner Maheesh Theekshana got big-hitter David Wiese caught behind for nought but Frylinck, who hit four fours in his 28-ball innings, and Smit had other ideas.
The pair took the attack to the opposition with five fours and two sixes in the final five overs.
Sri Lanka, who won the Asia Cup last month, lost Kusal Mendis for six when his top-edge off Wiese ballooned up to land in the gloves of Zane Green.
Ben Shikongo silenced the Sri Lankan fans when he sent back Pathum Nissanka for nine and Danushka Gunathilaka for nought on consecutive balls.
Bhanuka Rajapaksa played out the hat-trick ball, surviving a big lbw shout.
Frylinck, a left-arm seamer, then got Dhananjaya de Silva as Sri Lanka slipped to 40-4.
The left-handed Rajapaksa and skipper Dasun Shanaka, who made 29, attempted to put the chase back on track with a partnership of 34 but Bernard Scholtz broke the stand with his left-arm spin.
Scholtz got Rajapaksa out for 20 when he skied a shot and soon sent back Wanindu Hasaranga.
Frylinck ended captain Shanka’s resistance and the lower order soon followed to spark joyous celebrations when Wiese claimed the final wicket.
Earlier, SL skipper Dasun Shanaka had won the toss and elected to bowl first.
Sri Lanka were hit by an injury to left-arm quick bowler Dilshan Madushanka and brought in Dushmantha Chameera and Pramod Madushan.
“It’s a first game, we’d like to have a look at the wicket. The boys are very confident,” Shanaka said at the toss.
“We got the same combination that we had in the Asia Cup. With the bowling we have, we’d look to restrict them to a decent score.”
Namibia skipper Gerhard Erasmus said he would have also fielded had he won the toss. His side are looking to repeat last year’s feat of making the Super 12 on their T20 World Cup debut.
“Both captains would have liked to bowl first. The unknown factor of exactly how the wicket will play and a bit of overhead conditions,” said Erasmus.
The teams will each play three first-round matches in their attempt to make the Super 12 stage.
“It’s three games, three games of your life. You need to be on top from the start. If we can start well with the bat, put some pressure on them, that’ll go a long way into making it a very tight game.”
