The host nation's highest profile player won plaudits for his performances with Everton last season but as a result he felt his legs getting tired in the build-up matches and in Friday's opening game against Mexico in Group A.
"I played a lot of games in the last months and my legs are tired but as long as my mind and my heart are willing, I'll keep pushing myself as hard as I can go," he told a news conference.
Pienaar said the demanding physical nature of the Premier League meant it was no surprise key players like Rio Ferdinand, Michael Ballack, Michael Essien and John Obi Mikel were missing from the World Cup after sustaining injuries.
"It's a league of high intensity and when you are coming off a tough season, it's normal that sometimes the body gives," said the creative midfielder who scored seven goals in 38 appearances in all competitions for Everton last term.
"But for a tournament like the World Cup, you want to give your best and you can't let your body hold you back. You just have to push on," he added.
Pienaar, whose light frame belies a high work rate, did not last the full 90 minutes of Friday's 1-1 draw with Mexico at Soccer City, running out of energy as the game came to a close.
But he is still expected to be a key influence on Wednesday when Bafana Bafana play their next match against Uruguay, who drew 0-0 with France in their opener, and expects a bruising clash with the South Americans in Pretoria.
"They are well organised at the back and we know they are going to be tough. The most important thing is to focus on our own game and make sure we don't lose. The loser of the match is going to have to play catch up in the group," said Pienaar.
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