Her pep talk to American athletes included the advice: "Try to have fun. Try to breathe a little" – adding that it was "otherworldly" to be here.
She also told them with a wry smile that it wouldn't be a bad thing if they won either.
CLICK ON PHOTO FOR FULL GALLERY
She certainly had fun later in the day at a children's event for Let's Move, her fitness campaign.
Watched by 1,000 youngsters drawn from US military families and others from deprived inner city areas the all action first lady took part in a tug of war and played football.
On the pitch was David Beckham, attending in his role as an Olympic ambassador, and curiously SpongeBob SquarePants.
She told her young audience: "The stories of these athletes remind us that being an Olympian is not just about winning a gold or setting a record.
"It's about pushing yourself and believing in yourself and refusing to give up. No matter what obstacles you face, keep pushing."
Later that afternoon Michelle was due to be the guest of the Queen at a Buckingham Palace reception for VIPs around the world.