Defending champions Iran and Russia booked a repeat of the previous final as both won their last-16 ties at the International Volleyball Federation Boys' Under-19 World Championship in Tunisia.
The first day of knock-out play also featured a major shock as six-time record winners Brazil were dumped out in five sets by Bulgaria.
Iran, who beat Russia 3-1 in the 2017 final in Bahrain, continued their quest for a third title in all by defeating Cuba in straight sets.
The top seeds claimed a 25-19, 26-24, 25-22 victory at the Salle Polyvalente Radès.
Russia, the third seeds, kept their hopes of a fourth crown alive by beating European under-18 champions Germany 25-11, 25-20, 25-20.
South American age group champions Brazil, the seventh seeds, were left stunned as Bulgaria pulled off a 25-23, 24-26, 19-25, 25-21, 15-6 upset.
The Europeans finished fourth in Pool B to creep into the last-16 with just one victory but twice came from behind in a thrilling contest.
They will now meet Argentina in the quarter-finals after they rolled past United States 25-20, 25-19, 25-14.
Third seeds and Asian under-18 champions Japan had to come from behind to beat South Korea, the fourth seeds, 22-25, 25-19, 25-18, 25-17.
Their reward is a clash with Italy who knocked out Chinese Taipei in straight sets, 25-7, 25-17, 25-22.
Fifth seeds Egypt won an all-African clash with Nigeria, coming through 25-16, 25-20, 29-27.
It sets up a quarter-final clash with Belarus who accounted for sixth seeds Czech Republic with a 25-23, 25-20, 25-20 victory.
The quarter-finals will all take place tomorrow.