Rafael Nadal's drive towards a 17th Grand Slam title came to a shuddering injury-induced halt Tuesday on a day of Australian Open upsets that saw unseeded Kyle Edmund and Elise Mertens make the semi-finals.

The world number one retired against Marin Cilic after a muscle in his upper right leg began troubling him in 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, who battled past veteran Spaniard Carla Suarez Navarro in three sets, 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, who was beaten in last year's final by Roger Federer, 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," said the top seed, adding that it was too soon to say how bad the injury might be.

"I am a positive person, but today is an opportunity lost to be in the semi-finals of a Grand Slam and fight for an important title for me."

Croat Cilic, the sixth seed, said it was "sad" to win in such a way but added that "I played great tennis, very high level".

"So I am extremely pleased with the performance."

Nadal's retirement follows the departure on Monday of Novak Djokovic, with the Serb's 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. "It was my first match on Rod Laver Arena and it's very special."

He is the only British man in the draw after Andy Murray's injury withdrawal before the tournament, raising the prospect that it will be him, rather than the Scot, who breaks through to win in Australia.

Murray has been a five-time finalist, but lost them all.

- Too physical -

Source: AFP