Heats in various boat classes took centre stage in sweltering conditions at Nathan Benderson Park where temperatures reached a humid 34 degrees Celsius.

Single sculler Kallfelz, who also has a bronze in this class, won her race by almost seven seconds to advance directly into the final and avoid the need for a repechage.

It means she is on course to upgrade the second-place finish she achieved in Poznań in Poland in 2018.

Australia's Ria Thompson won the other heat to also book a place in the final with only one rower going through automatically from each race.

In the men's single sculls the top four from each heat advanced to the quarter-finals with Greece's Stefanos Ntouskos, a Rio 2016 Olympian in the lightweight four, among the race winners.

Returning silver medallist Marc Weber of Germany also won his heat while Australia's Youth Olympic bronze medallist Cormac Kennedy-Leverett made it through behind Ntouskos.

In the men's pair, South Africa's defending champion Charles Brittain will have to go through the repechage with new partner Luc Daffarin.

With only one boat from each heat going through to the final they finished behind Povilas Stankunas and Mantas Juskevicius of Lithuania.

The women's pair saw the top three boats in each heat reach the semi-finals with Greece's Youth Olympic champions and world junior gold medallists Christina Bourmpou and Maria Kyridou among those to win.

Three semi-final spots were also available in each men's quadruples sculls heat with Britain, returning with three of the rowers who won the event last year, winning their race.

Hosts the US also celebrated qualifying their women's pair of Hadley Irwin and Sarah Johanek to the semi-finals as five of their crews went through across the classes while avoiding the repechage in all.

Both Irwin and Johanek won medals at last year's event but in different boats.

"I think we definitely got the job done, we came out to lay down a fast and solid first race, and I think that we did what we needed to do," said Irwin. 

"There's definitely a lot of tough competition out there but we're looking forward to continuing to race and getting faster every day."

More heats are due to be held tomorrow before repechages and semi-finals on Friday (July 26).

Two days of finals will be held to conclude the event.