Lithuanian teenager Ruta Meilutyte provided her older and more experienced Olympic rivals with an ominous warning of her intentions when she topped the qualifiers of the women's 100 meters breaststroke heats.
The 15-year-old, who was competing at European Youth events just a year ago, showed no signs of nerves on the big stage as she easily won her heat in one minute 05.56 seconds.
Australia's Leisel Jones, the defending Olympic champion who won a silver medal at the 2000 Sydney Games when she was 15, finished second in the heat and fifth overall.
"I am pretty happy with that - probably one of the best heat swims I have done," said Jones. "You can't afford to relax. Most of the girls have gone pretty hard this morning. You can't afford to go easy and miss the semis."
American Rebecca Soni, the world champion and favorite to win the gold, was second overall in 1:05.75.
"It feels good to finally get a race in," said Soni. "I am not focusing on any pressure, or on defending any titles. I am just going to have fun and race."
Soni said she was impressed by Meilutyte.
"It's great to see when someone swims faster than they thought they could," said Soni.
"That joy rubs off on the rest of us."
(Reporting by Julian Linden; Editing by Greg Stutchbury)