Tatjana Schoenmaker broke the 200m women's breaststroke Olympic record on Wednesday but says she never expected to topple the world record in her gold medal swim on Friday.

The South African swimmer won the gold medal ahead of USA's Lilly King and Annie Lazor in the final, setting a world record 2:18:95 in the process.

King went out fast, but at the final turn Schoenmaker was marginally ahead before motoring clear.

Schoenmaker had broken the Olympic record on Wednesday with a time of 2:19:16 in her heat before surpassing the mark held by Denmark's Rikke Moller Pedersen of 2:19:11 from August 2013 in Barcelona.

"It'll kick in when I get home probably," Schoenmaker said. "I wasn’t expecting that at all.

"I was really trying to focus on my own race. [King] definitely pushed me, knowing that her first 100 is so good.

"It was so amazing to be able to race today. It was such a tough race. We were all racing to do well. It just still hasn’t sunk in, maybe one day."

Schoenmaker was visibly emotional post race, celebrating with shock after realizing she had broken the world record, with Lazor embracing her in the pool as the South African came to tears.

"I feel like it really hasn’t sunk in yet," Schoenmaker. "The whole process went really quick. I feel I was very excited to also finish my individual races.

"I was excited to go home and see my family. I don’t know if it should be kicking in now. If I was crying that much then, I can't imagine."

Schoenmaker becomes the third woman to win an Olympic gold medal for South Africa in swimming, after Penelope Heyns (two in 1996) and Joan Harrison (1952).

She also becomes the fourth South Africa woman to swim a world record, behind Ann Fairlie (100m backstroke), Karen Muir (100m and 200m backstroke) ad Heyns (50m, 100m, 200m backstroke).

"Penny [Heyns] was amazing on her own," Schoenmaker said. "She's created such an amazing name in the swimming environment. To even be close to her is so amazing."