Dominic Thiem shocked top seed Rafael Nadal and Alexander Zverev ousted former champion Stan Wawrinka as tennis's young pretenders stole a march on the old guard to reach the Australian Open semi-finals on Wednesday.

Austria's Thiem, 26, beat Nadal 7-6 (7/3), 7-6 (7/4), 4-6, 7-6 (8/6) - his first win in six attempts at Grand Slams, including two French Open finals - to end the 33-year-old's bid to equal Roger Federer's record 20 Major titles.

Earlier Zverev, 22, beat 2014 champion Wawrinka, 34, to go into a semi-final with Thiem, meaning that one of them will reach Sunday's final against Roger Federer or Novak Djokovic, who have won 12 of the last 14 Australian Open titles.

Thiem had Nadal's measure in the first two sets and he recovered from dropping the third to win the fourth-set tie-breaker, getting off the floor after falling to win one point and enjoying some luck from the net cord to set up the third and decisive match point.

"I was lucky in the right situation – the net cord was really on my side," Thiem said. "But it's necessary because he is one of the greatest of all time. You need some luck to beat him."

Several young players have been knocking on the door of the Big Three of Nadal, Federer and Djokovic, and either Thiem or Zverev will get a chance to kick it open in Sunday's final.

'Almost too focused'

Germany's Zverev rallied from a set down to halt three-time Major-winner Wawrinka 1-6, 6-3, 6-4, 6-2 and reach his first Grand Slam semi-final.

Big things have been tipped for Zverev since he burst into the top 10 in 2017, but he said that until now he has been pushing himself too hard at the Majors.

"I was doing things in a too professional way. I was not talking to anybody. I wasn't going out with friends. I wasn't having dinner. I was just really almost too focused."

"I changed that a little bit this week... maybe this is how it should happen," he added.

Wimbledon champion Simona Halep has also benefited from a more relaxed approach as she swept aside Anett Kontaveit 6-1, 6-1 to reach the semi-finals without dropping a set.

The Romanian fourth seed, who is spending time away from tennis between matches, said she was finally feeling comfortable on the biggest stage.

"I just feel more confident and I feel like I'm able to do it," said Halep, who is going after her third Major title. "It's just a feeling that you don't see this trophy as impossible anymore."

Halep's semi-final is against fellow two-time Slam-winner Garbine Muguruza, who continued her impressive return to form by beating Russian 30th seed Anastasia Pavlyuchenkova 7-5, 6-3.

"I'm excited to play my first semi-final here," said the unseeded former world number one, who won the 2016 French Open and 2017 Wimbledon.

"I've known her (Halep) for quite a long time so it's going to be a tough match," she added.

Meanwhile, Roger Federer was fined 3,000 U.S. dollars for using an audible obscenity during his Australian Open quarter-final victory over American Tennys Sandgren.

The Swiss was handed a code violation for swearing in the third set of Tuesday's match after the lineswoman reported Federer to Serbian chair umpire Marijana Veljovic.

Federer clarified in his post-match news conference that he had sworn in a "a mix" of languages and said he was frustrated by the way he received the on-court warning.

Federer went on to save seven match points against Sandgren before winning in five sets to set up a semi-final clash with defending champion Novak Djokovic.