Bol, took the lead with 250 metres to go in the opening heat on Saturday and held his form all the way to the line, crossing second behind Kenya's Ferguson Rotich in one minute 44.13 seconds.

The previous national mark of 1:44.21 was set by Bol's close friend and training partner Joseph Deng three years ago in Monaco.

The Sudanese-born Bol left it late before booking his spot on the Australian Games team at a meet in Queensland in June.

"I've been chasing times and I've been chasing this top two - that's what you need at an Olympics," said the 27-year-old, who was eliminated in the heats on his Olympic debut in Rio five years ago.

"It's not about times today, it's more about the positions.

"Credit to my boy Joseph Deng for holding that record."

Bol will be aiming for a similar showing in Sunday evening's 800m semi-finals.

"Same thing," he told the Seven Network.

"Race strong, race hard, top two.

"Try and get into the final and then anything can happen."

Bol was the second fastest overall qualifier behind Rotich.

Australia will have three men in the 800m semis for the first time at Olympic level after Jeff Riseley and Charlie Hunter also advanced.

Competing in his fourth Olympics, Riseley - a training partner of Bol - was fourth in his heat in 1:45.41 while Hunter clocked 1:45.91.

The men's 800m final will take place on Wednesday.

The last Australian man to contest an Olympic 800m final was Ralph Doubell, who won gold in Mexico City in 1968.

Liz Clay made an impressive Olympic debut, cruising into the 100m hurdles semis by finishing second in her opening-round heat in 12.87 behind world record holder Kendra Harrison from the US (12.74).

The late-blooming Clay is coached by Sharon Hannan, who also mentored Sally Pearson for much of her storied career.

Australian Sarah Carli was eliminated in the women's 400m hurdles after finishing fifth in her heat in 56.93.

Australian Associated Press