TOKYO: Brazil's Marcela Ana Cunha said she had to be "cold mentally" to see off the challenge of defending champion Sharon van Rouwendaal on her way to Olympic gold in the women's 10-kilometre marathon swim on Wednesday.

Cunha won by less than a second from a fast-charging Van Rouwendaal in their early morning duel in Tokyo, timing 1hr 59min 30.8sec to 1:59:31.7 for the Dutch swimmer.

Kareena Lee took the bronze medal for Australia.

Cunha, 29, finished 10th in the event at the Rio Olympics and fifth at the Beijing Games in 2008.

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The Brazilian said she had arrived in Japan desperate to win gold.

"We arrived here wanting, as much as you can, this medal and around 10 days ago, I said to my coach to win this race will be very difficult for my opponents because I want it so hard, so much and I'm really well prepared," she said.

"We had to be cool or 'cold' as Europeans (are)," she added. "We are Latin people -- we are hot, we are emotional people -- so I had to be very cold mentally in the race to be focused and I had to win it myself. I knew I was prepared for that."