Australian success at the swimming world championships is driving on Olympic 200m breaststroke champion Zac Stubblety-Cook, but he cautioned Thursday against getting "overexcited" by his teammates' achievements.

Australian swimmers have won six gold medals and broken three world records in four days of competition so far in Fukuoka.

Ariarne Titmus started things off when she won the women's 400m freestyle in a world record time on the competition's opening night, before Australia's women's 4x100m freestyle team also set a new mark.

Mollie O'Callaghan then beat Titmus in the women's 200m freestyle on Wednesday to break a world record that had stood since 2009.

Stubblety-Cook is hoping to emulate that success but warned that "you can get caught up" in the mood in the Australian camp.

"You can get a bit overexcited by it all but you see people like Arnie (Titmus) step up and do what they do every day, and Mollie as well," he said.

"You definitely feed off that, for sure.

"That is exciting but you also have to put it in perspective and go back to that process and get the best out of yourself."

Stubblety-Cook is the world record-holder in the 200m breaststroke and he qualified from the heats in that event with a fastest time of 2min, 08.98sec.

Caspar Corbeau of the Netherlands was second-quickest on 2:09.29, followed by American Matt Fallon on 2:09.32.

China's Qin Haiyang, who won both the 50m and 100m breaststroke titles in Fukuoka, qualified fifth-fastest in a time of 2:09.86.

Stubblety-Cook said it was "very rare" to see a swimmer with Qin's talent.

"It's exciting because he obviously has a very different race plan to a lot of the 200 guys," he said.

- 'Surreal feeling' -

Stubblety-Cook is aiming to defend the 200m breaststroke world title that he won last year in Budapest.

That came hot on the heels of setting a new world record of 2:05.95 at the Australian trials the previous month.

The 24-year-old said it was "still a surreal feeling" to see his name beside the world record.

"I never thought that would actually happen, and I do have to put it into perspective and just step back," he said.

O'Callaghan was back in the pool the morning after breaking the 200m freestyle record in a time of 1min, 52.85sec.

The 19-year-old qualified seventh-fastest in the 100m freestyle heats, coming home in 54.01sec in an event in which she is the defending champion.

Hong Kong's Siobhan Haughey was fastest on 53.15, followed by American Abby Weitzeil on 53.25 and Australia's Emma McKeon on 53.40.

In the men's 200m backstroke, Australia's Bradley Woodward qualified as top seed from the heats in a time of 1min, 57.14sec.

Switzerland's Roman Mityukov was second-fastest on 1:57.24, followed by Hungary's Hubert Kos on 1:57.27.

Tes Schouten of the Netherlands was quickest in the women's 200m breaststroke heats, coming home in 2min, 22.43sec.

She was followed by 100m breaststroke silver-medallist Tatjana Schoenmaker of South Africa on 2:22.92 and Denmark's Thea Blomsterberg on 2:23.41.

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