World bronze medallists Serbia whipped Thailand 25-16, 25-23, 25-15, while Japan outplayed Dominican Republic 25-21, 18-25, 25-16, 25-21 to remain unbeaten in the eight-team round robin competition.
The winners of the tournament, along with the best-placed Asian nation and the two other best-placed teams from the remaining six, qualify for the Beijing Games in August.
Poland trailed them with a 2-2 win-loss record, followed by Thailand, Dominican Republic and Puerto Rico with 1-3 and Kazakhstan 0-4.
The tournament combined the Asian qualifying round.
Kazakhstan, meanwhile, were almost knocked out of competition when they suffered a 25-19, 15-25, 25-22, 23-25, 15-13 defeat to Puerto Rico in their fourth straight loss.
South Korea suffered another setback when they crashed to two-time European champions Poland 25-16, 25-19, 25-16.
—AFP