Forward was ailing.
The team practiced at Eersterust Stadium, where practice was moved because FIFA wanted to preserve the condition of the Loftus Versfeld field, where Algeria and the USA will play Wednesday's closing Group C game.
Altidore did join the squad as the team practice was closed to media.
The striker has yet to score in the World Cup, but it was his knock-down header for an assist to Michael Bradley that let to the tying goal for the Americans versus Slovenia.
"Jozy just wasn't feeling well, but he'll be fine for tomorrow," U.S. coach Bob Bradley said.
If Altidore is unable to play, Edson Buddle may deputize as a starter. Herculez Gomez is another option at forward.
Robbie Findley is suspended after getting yellow cards against England and Slovenia.
The 2010 World Cup is finally here, so keep up to date with all the news at Goal.com's World Cup homepage and join Goal.com USA's Facebook fan page!