Kaylee McKeown completes backstroke double with 200m gold

The world record holder once again overhauled hapless American challenger Regan Smith to claim the win in an Olympic record 2:03.73 at La Defense Arena.

McKeown joined a select group of swimmers with four Olympic gold medals in individual events and is the first Australian athlete in any sport to win four.

She said it was something she could never have imagined.

"Not in a million years," she added.

"Growing up I've always just idolised my sister and (swimmer) Emily Seebohm and seeing what they do in the sport and then training alongside Emma McKeon as well, one of the greats in the sport and being friends with Molly (O'Callaghan)," she told reporters.

"I couldn't ask for much more to be honest with you, having that motivation and seeing those girls."

Smith, more than a half-second adrift, took her third silver of the Games, denied gold for a second time by the same Australian dominator who beat her for the 100m title on day four.

Kylie Masse took bronze for Canada, her fifth Olympic medal, three years after earning silvers in both the 100 and 200m at Tokyo.

The Olympic women's backstroke has witnessed a procession of outstanding swimmers including Krisztina Egerszegi and Missy Franklin since the 200m joined the 100m event at the 1968 Olympics.

But the improbable "double-double" eluded all of them until now.

McKeown is the second swimmer to pull it off and the first in more than 50 years, with Roland Matthes, the East German "Rolls Royce", going back-to-back in both events in 1968-1972.

A predatory swimmer habitually stalking from behind, there was some surprise when McKeown went hard off the blocks and led for most of the first 50m.

"I probably took my race out a little bit too hard. I was pretty nervous going in there tonight," she said.

"I'm not one who gets overly nervous, probably more anxious than anything."

She settled into her familiar hunting ground, though, in the middle of the pack.

Smith should know better by now than to try to outlast McKeown from the front -- but that's exactly what she tried to do.

The 22-year-old from Minnesota shot to the lead by the second length and held it into the last before her recurring nightmare unfolded as the relentless McKeown picked her off again.

McKeown is now Australia's first Olympian to win four gold medals in individual events as opposed to team ones, taking her past a parade of iconic names like Dawn Fraser, Ian Thorpe and Betty Cuthbert.

Her triumph was Australia's seventh gold of the meet and came only minutes after teammate Cameron McEvoy stormed to the men's 50m freestyle title.

"I went out hard and just held on for dear life," said McKeown, who will likely be selected for Australia's relay team in the mixed medley final on Saturday.

"My main events are out of the way and tomorrow is just a fun time for me."