Outsider Bianca Buitendag eliminates world no. 5 Stephanie Gilmore in women's Olympic surfing

Rank outsider South Africa's Bianca Buitendag says she "had nothing to lose" after beating world no. 5 Stephanie Gilmore. Ryan Pierse/Getty Images

Australia's hopes of a surfing gold medal suffered a crushing blow as world no. 5 Stephanie Gilmore crashed out of the women's competition, losing to rank outsider Bianca Buitendag of South Africa in their last 16 heat early on Monday.

Though the waves were bigger and more powerful than those on offer during Sunday's opening session, choosing which one to ride proved difficult and Gilmore, who posted the best scores on the opening day, came up short in her selections as Buitendag swept to victory.

Few expected Buitendag to have any chance at all against the decorated Australian, but the lack of expectations allowed her to perform at her best as she won by a score of 13.93 to 10.00.

"I had nothing to lose, absolutely no pressure. I am the underdog coming in at 17th seed for this event. I had nothing to lose and it's a really comfortable spot to be in," the beaming 27-year-old told reporters.

"It was very hard (to pick the right waves), but I knew I was up against Steph, who is the best women's surfer in the world so I knew I had to be on the best waves, otherwise I stood no chance."

Knowing that Gilmore could pull out a winning run at any time, the closing stages of the half-hour heat were tough for Buitendag.

"It's nerve-wracking. I know she can get a ten (score) at any given moment, so that was probably the longest three minutes of my life!" Buitendag said.

Judged on the combined scores from their two best waves, surfers in head-to-head heats are given priority when choosing which ones to ride.

When Gilmore passed up her chance on a promising wave Buitendag grabbed the opportunity, using it to post a score of 7.10 that proved pivotal.

"That's just the nature of surfing, sometimes the waves are there, sometimes the waves are not," seven-time world champion Gilmore told reporters.

"I looked at that wave and I was like, it doesn't look that good, so I let her have it and she turned it into a seven, so that was the most frustrating thing to me - like, man, I should have just taken that wave."

With bigger swells in the ocean and higher onshore winds, local Olympic volunteers had to remove some parasols and tents to stop them blowing away.

The competition continues with 32 athletes in total taking part in Monday's knockout competition.