Melbourne (AFP) - Carlos Alcaraz sent a chilling warning to his Australian Open rivals on Saturday, outclassing Shang Juncheng before the Chinese teenager was forced to retire as Iga Swiatek aims for her 19th straight win.

A day after defending champion Novak Djokovic returned to top form, world number two Alcaraz laid down a marker in a lopsided third-round contest lasting just 66 minutes.

The 20-year-old, playing a tour-level match against a player younger than himself for the first time in his career, was leading 6-1, 6-1, 1-0 when his 18-year-old opponent had to pull out.

Shang entered the match on Rod Laver Arena with his upper right thigh strapped and received treatment on the injury during the second set.

Dominant Alcaraz did not face a single break point to set up a last-16 meeting with Serbia’s Miomir Kecmanovic, who saved two match points to beat last year’s semi-finalist Tommy Paul 6-4, 3-6, 2-6, 7-6(⁹⁄₇), 6-0.

The Spanish second seed is vying with Djokovic for the number one ranking and if they both reach the final it will double as a straight shootout for the top spot.

Alcaraz, who missed last year’s tournament at Melbourne Park with an injury, said it was “not the way anybody wants to move on” after the injury to Shang.

“I missed the tournament last year,” he said. “I was watching the matches at home from the couch, wishing to play in the second week here.

“This is the first time I have moved to the second week in Australia. It feels special.”

In other action on Saturday, women’s 12th seed Zheng Qinwen said she was inspired by Chinese great Li Na’s win in the 2014 final after she beat compatriot Wang Yafan 6-4, 2-6, 7-6 (¹⁰⁄₈).

“That final I have watched it more than 10 times, so I have a lot of memories,” she said.

“I mean, she was an unbelievable player, the first Asian woman who won the Grand Slam. How can you not remember that.”

Men’s ninth seed Hubert Hurkacz progressed but former US Open champion Sloane Stephens was beaten by Russia’s Anna Kalinskaya.

- Swiatek streak -

Top women’s seed Swiatek is currently on an 18-match winning run, but she had to dig deep to see off former finalist Danielle Collins in the second round.

On her days off she has witnessed defending champion Aryna Sabalenka pummel her three opponents into submission, for the loss of just six games in total.

The 22-year-old will be keen for a statement win against unseeded Czech Linda Noskova to confirm that she is the player to beat as she hunts a fifth Grand Slam title.

Swiatek, who faced former Australian Open champion Sofia Kenin in the first round, said her draw was “like the toughest one I ever had”.

But she believes she could benefit from her baptism of fire in Melbourne.

“I don’t mind actually that it was intense because I remember also when I started the tournament with some easy matches, then I felt I was a little bit rusty when the tough moments came,” she said.

Men’s third seed Daniil Medvedev is back on court after his gruelling five-set encounter with Finland’s Emil Ruusuvuori, which finished at 3:40 am on Friday.

The Russian, a two-time finalist in Melbourne, faces a potentially tough assignment against Canadian 27th seed Felix Auger-Aliassime, while sixth seed Alexander Zverev takes on unseeded American Alex Michelsen.