Along with Mathews, De Silva also showed glimpses of his character by scoring a 63-run knock before getting dismissed off Victor Nyauchi.
Dhananjaya de Silva scored a gutsy 57, but waged a lone battle for Lanka in a stiff run-chase.
The Proteas will be looking forward to ending this season with one last win.
Three batsmen from the hosts' scored half-centuries.
Sri Lanka will want to produce their A game in the tournament opener and set the tone for the rest of the matches to follow.
Dhananjaya de Silva tried to make a meal out of the chase with his 84-run knock but it was too late for the hosts.
The 26-year old Lankan batsman stayed back in the country after being named in the 17-man squad for the Caribbean tour.
The brutal assassination of his father has sent shock waves in the country.
All the stats and numbers that were recorded during the run feast day in Chittagong.
Kusal Mendis missed out on his double ton by just 4 runs as Sri Lanka ended the day on 504/3.