Rafael Nadal's drive towards a second Australian Open title came to a shuddering injury-induced halt Tuesday on a day of upsets that saw unseeded Kyle Edmund and Elise Mertens make the semi-finals.
The world number one retired against Marin Cilic after an upper right leg problem began troubling him the fourth set on Rod Laver Arena, with the Spaniard wincing in pain and limping as he struggled to continue.
His 3-6, 6-3, 6-7 (5/7), 6-2, 2-0 exit set up a last-four clash for the former US Open champion against Britain's Edmund, who stunned third seed Grigor Dimitrov 6-4, 3-6, 6-3, 6-4.
Mertens, who is yet to drop a set, was equally convincing in blasting past world number four Elina Svitolina 6-4, 6-0 to become the first Belgian to make the semis since Kim Clijsters in 2012.
She will play second seeded Dane Caroline Wozniacki or veteran Spaniard Carla Suarez Navarro for a place in Saturday's final.
Little separated Nadal and big-hitting Cilic until the injury struck, as they traded ferocious groundstrokes in an intense battle.
But the Spaniard called the physio at 1-4 in the fourth set and again at the changeover when two sets apiece and the writing was on the wall.
"Tough moments. Is not the first time an opportunity that is gone for me," Nadal told reporters. "I am a positive person, but today is an opportunity lost to be in the semifinals of a Grand Slam and fight for an important title for me.
"This has already happened to me a couple of times in this tournament and it's really tough to accept, especially after a tough December when I missed starting the season in Abu Dhabi and then Brisbane."
Nadal went into the year's opening Grand Slam tournament with knee concerns, but ironically that wasn't the problem that finally brought him undone.
"I was playing okay. I was playing a match that anything could happen: could win, could lose. He was playing good, too," Nadal said.
"But I was fighting for it. I was two sets to one up. Yeah, just have to accept, recover, go back home, stay with my people and keep going. That's all."
Nadal said his problem was not hip-related, more a muscle issue high up on his right leg.
"It was an unbelievable performance from both us," said Croat Cilic, the sixth seed. "It is really unfortunate for Rafa to finish this way."
Nadal's retirement follows the departure on Monday of Novak Djokovic, with his immediate playing future uncertain after an elbow injury flared. He also appeared to have a hip problem.
There were no such troubles for Edmund against Dimitrov, as he became only the fourth British man to reach the Australian Open semi-finals in the post-1968 Open Era.
"It's an amazing feeling. I'm very happy," said the overwhelmed 23-year-old, ranked 49.