Vieri said that the Italians were still tired after a long league season, which was not resolved until the final game, when three teams were still in the running for the Serie A title.
"Our league is a real league. All the games on Sundays are battles.
"We're all in general a little tired. The other teams are definitely a bit fresher than us."
This is not a problem for the South Koreans, who have impressed onlookers with their tireless running in their games so far.
US coach Bruce Arena, whose team faced the Koreans in the first round, said: "A country like
Korea basically has trained their national team for three or four months, and they could win a track meet right now they are so fit.
"Other countries don't have that opportunity."
Italy have more worries than simply the fitness of the South Koreans - key central defender Alessandro Nesta is an injury doubt for the game.
Nesta missed training on Sunday and is doubtful for the game because of a badly bruised foot.
The Manchester United target was in a similar position before Italy's previous game against Mexico, but he came through to play.