Outside hitter Bianka Busa swept a game-high 26 points for the title holders with 20 from attacks, four from blocks and two aces. Captain Tijana Boskovic added 22 and Sara Lozo had 16.
Currently ranked 15th in the world, Bulgaria got better of the world No. 5 in the second and third sets. Five players hit double-digit scores for Bulgaria with opposite Silvana Chausheva leading the scoring at 18 points.
But Serbia recovered in the last two sets, especially in the decider, as Bianka contributed four points with an efficiency of 50 percent.
Serbia slipped to third place in the pool after the win, falling behind the United States and Germany. Bulgaria had one point to rank fourth while Canada and Kazakhstan, who both remain scoreless, sit at the bottom of the pool.
The top four teams from the four pools advance to the next phase of the tournament.
Olympic champions the United States claimed their second straight-set win by outscoring Canada 25-19, 26-24, 25-15 in the last match of the day. Earlier in the day, Germany had tamed Kazakhstan, also in straight sets 25-15, 25-18, 25-21.
For a second straight match, Ali Frantti led the American offense. The 26-year-old outside hitter chipped in 15 points against Canada and hit an impressive 52 percent efficiency rate.
“At the end of the day, we’re just having fun,” Frantti said. “We’ve been pushing so hard in training leading up to the World Championship and we know every team will come here and do their best. They pushed us at the end of the second set but we were ready to battle for each point and went for it.”
Olympic champion opposite Annie Drews, who had a 50 percent efficiency rate in hitting, came right behind Frantti with 14 points.
In Pool D, Japan cruised past the Czech Republic 25-15, 25-20, 25-12 to share top spot with Brazil, who also wrapped up their second straight-set victory by beating South American neighbors Argentina 25-19, 25-13, 25-21.
China, another powerhouse in Pool D, will take on Colombia on Tuesday.
