Australian Open triggers maximum heat warning with 45C forecast
The Melbourne major reached the highest level of five for the second time in four days on its scale designed to protect players, fans and officials.
That means centre court, Rod Laver Arena, which hosts the day's four quarter-finals, closed its roof.
The opening quarter-final of the day, between Aryna Sabalenka and Iva Jovic, took place in blazing sunshine.
The match, won by world number one Sabalenka, ended in temperatures of 38C.
Play on outdoor courts, which were to host junior boys' and girls' matches, were put on hold.
Temperatures reached 40C on Saturday, with matches on outside courts paused for about five hours.