FIFA is investigating why a section of seats in the Soccer City stadium was unoccupied during the World Cup opening match between South Africa and Mexico.
"There was one block that looked empty and our people in ticketing are looking into it," FIFA spokesman Nicolas Maingot said Saturday.
The upper tier section of around 200 seats was filled by tournament volunteers later in Friday's match, which ended 1-1. FIFA said 84,490 people attended at Soccer City - a record for one of its matches in Africa.
All tickets have been sold for Saturday's matches between England and the United States at the 45,000-capacity Royal Bafokeng stadium in Rustenburg, and at 62,500-capacity Ellis Park in Johannesburg where Argentina faced Nigeria, FIFA said.
However, the 46,000-capacity Nelson Mandela Bay stadium in Port Elizabeth was only about two-thirds full for South Korea's 2-0 win over Greece. FIFA said only 3,000 tickets were still available early Saturday, but that many ticket-holders simply didn't show up for the game.
A total of 31,513 spectators was declared as the official attendance inside the stadium.