The win kept alive Croatia's chances to advance to the quarterfinals.
Croatia (1-3) led 41-32 at the half and scored the first 12 points in the third quarter to put the game away. Lelas had five points during the burst.
Luisa Tomas scored 15 points and Nadir Manuel added 14 to lead Angola (0-4), which could only get within 13 the rest of the way.
African nations have only won one of their 27 games in the Olympics since Congo — formerly known as Zaire — first qualified in the 1996 Atlanta Games.
The Croatians play Turkey on Sunday; Angola faces the Czech Republic.