Plíšková booked her place in her first Australian Open semi-final as she battled to a 6-4, 4-6, 7-5 victory in a thrilling contest on Rod Laver Arena.

Williams was in a commanding position to progress when she led 5-1 in the deciding set but Plíšková mounted a stunning fightback to shock the 37-year-old American.

The seven-time Australian Open champion served for the match at 40-30 but appeared to suffer an ankle injury and did not win another point on serve throughout the remainder of the set.

Williams insisted she did not "choke" during the decider, claiming she "just played lights out on match points".

Plíšková will next face US Open champion Naomi Osaka - who beat Williams in the controversial final - after the Japanese star claimed an impressive 6-4, 6-1 win over Ukraine's Elina Svitolina.

"I was almost in the locker room but now stand here as the winner," said Plíšková.

"My mind was in the locker room at 5-1 down but I was still here. 

"I was too passive and mentally down but she got a little bit shaky.

"Naomi Osaka is dangerous but there is nobody more dangerous than Serena."

In the men's draw, six-time Australian Open winner Novak Djokovic reached the last four after opponent Kei Nishikori retired in the second set of their eagerly-anticipated clash.

Djokovic led 6-1, 4-1 when the Japanese player, who needed five sets to oust Spaniard Pablo Carreño Busta in the previous round, retired with a thigh problem.

Frenchman Lucas Pouille awaits for Djokovic in the last four after he overcame 16th seed Milos Raonic of Canada.

Pouille, who had failed to win any previous Australian Open singles match until this year, beat the Canadian 7-6, 6-3, 6-7, 6-4 to book his place in his first Grand Slam semi-final.

The omens are in Djokovic's favour as he has won the tournament every time he has made it to the semi-finals.