Extremely rare change made to Olympic semi-final after 'extraordinary situation' occurs during heat

A rare change has been made to the semi-finals of an Olympic event after an 'extraordinary situation' occurred during the heats.

With so many elite athletes competing at the Olympic Games, events are often decided by the finest of margins.

That was certainly the case in the eagerly anticipated men's 100 metre final on Sunday evening, with America's Noah Lyles pipping Jamaica's Kishane Thompson to gold by five-thousandths of a second.

Sometimes the margins are so narrow that they... don't exist at all.

On Monday morning an incredible situation unfolded when two athletes couldn't be separated at the finish line of the women's 400m hurdles.

In Heat 1 of the repechage round - a recently introduced extra round for athletes who fail to qualify for the semi-finals through the standard heats - Naomi van den Broeck of Belgium and Alanah Yukich of Australia finished in a dead tie.

Both women crossed the finish line in exactly 55.107 seconds, behind winner Ayomide Folorunso of Italy who clocked a time of 55.07.

Organisers were left with a problem, as only the top two in each of the three repechage heats progress to the semi-finals.

The solution? Send both women through anyway.

To make sure that neither has to miss out, lane one - which is not usually used in races - will be used to make more room on the track to accommodate both.

CBS Sports reporter Devin Heroux described Monday morning's tie as an "extraordinary incident," that caused "the longest delay we’ve had here so far."

The semi-finals will take place on Tuesday, with the final of the women's 400m hurdles scheduled for Thursday.

Sydney McLaughlin-Levrone and Femke Bol are the two favourites to win the final - provided they qualify - as they are they only two women in history to clock a sub-51 second time in the event.

Both qualified for the semi-finals with ease.