FIFA is investigating whether transport and ticket distribution systems failed after at least 8,000 seat-holders did not attend the World Cup match between South Korea and Greece in Port Elizabeth.
The problem appeared to occur for a second straight day Sunday, as another match between two of the tournament's less glamorous sides — Algeria and Slovenia in Polokwane — kicked off at 1:30 p.m. local time with about one-third of seats empty.
Only 31,513 people attended South Korea's 2-0 victory on Saturday at the Nelson Mandela Bay Stadium, which holds 42,486 spectators for World Cup games.
FIFA had said just 3,000 tickets were available on the morning of the match.
"It looked like a lot of people did not show up at the stadium. We are investigating this," FIFA spokesman Nicolas Maingot said Sunday.
Before the tournament, FIFA aimed to ensure full stadiums by offering reduced-rate tickets for matches which proved less attractive to fans in the worldwide sales process.
Host cities were offered group deals to allocate seats among local businesses and public service workers.
Maingot said FIFA helped pay for bus transport to bring South African fans to the lower-demand games.
"We asked for a system to be put in place. Apparently, that has not completely worked," he said. "We are working on this to make sure this is not the case for a potential few others of those games."
FIFA said Sunday that 700 unsold tickets were available hours before Algeria and Slovenia played in the 41,733-capacity Peter Mokaba Stadium.
An official attendance of 30,235 was declared.
Transport has been a difficult issue in the early days of a World Cup that is proving a popular success with host nation fans and visitors.
Organizing committee officials experienced what they called "challenges" getting 84,490 fans to a sold-out Soccer City in Johannesburg on Friday for the opening ceremony and South Africa's first match against Mexico.
World Cup and city officials were working Sunday to create a more efficient transport plan before the stadium hosts the well-supported Netherlands against Denmark in Monday's lunchtime kickoff.