TOKYO: US teenager Lydia Jacoby stunned team-mate and defending champion Lilly King to claim the 100m Olympic breaststroke gold medal in Tokyo on Tuesday.

The 17-year-old swam a scintillating final 50m to touch in 1min 04.95sec and edge South Africa's Tatjana Schoenmaker (1:05.22) into second, with King having to settle for bronze in 1:05.54.

"It was crazy. I was definitely racing for a medal. I knew that I had it in me," said Jacoby.

"I wasn't really expecting a gold medal. So, when I looked up and saw that scoreboard I was insane."

Japan's Horigome crowned first skateboarding Olympic champion

Schoenmaker surprisingly started the race as the fastest qualifier after setting a new Olympic record in the heats.

And she looked destined for gold medal glory at the turn.

But Jacoby found some devastating speed to haul her in, leaving King in their wake.

"I'm surprisingly OK right now. I'm very happy with my race and so excited for Lydia," said King.

"I mean, I love to see the future of American breaststroke coming up like this."