Australia's Nick Kyrgios reacts as he announces his withdrawal from the Australian Open with a knee injury at a press conference in Melbourne, Australia on Jan 16, 2023. (MARK BAKER / AP)=

MELBOURNE - Home favorite Nick Kyrgios pulled out of the Australian Open with a knee injury on the eve of his first-round match in a huge blow to organizers after in-form Americans Jessica Pegula and Coco Gauff raced into the second round on Monday's opening day.

Wimbledon finalist Kyrgios revealed he had suffered a torn meniscus and decided to withdraw after a hitting practice session with doubles partner Thanisi Kokkinakis.

"Obviously, I'm extremely disappointed, this is one of the most important tournaments of my career, it hasn't been easy at all," Kyrgios told reporters at Melbourne Park.

"This coming around is just bad timing but that's life, you know. Injuries are part of the sport."

With home favorite Nick Kyrgios gone and Ash Barty retired, the chances of a another home champion of the Australian Open for local fans are greatly diminished

The withdrawal also ends Kyrgios and Kokkinakis's defense of the men's doubles title, a year after the "Special K's" drew massive crowds through their unlikely championship run.

With Kyrgios gone and Ash Barty retired, the chances of a another home champion for local fans are greatly diminished.

However, US fans were encouraged after seeing Pegula, seeded third behind Iga Swiatek and Ons Jabeur, needing less than an hour to see off Jaqueline Cristian 6-0 6-1.

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Pegula hit her stride early in the new season by helping the United States win the inaugural United Cup, and the 28-year-old simply overpowered her Romanian opponent on Monday.

Jessica Pegula of the US hits a return against Romania's Jaqueline Cristian during their women's singles match on day one of the Australian Open tennis tournament in Melbourne on Jan 16, 2023. (ANTHONY WALLACE / AFP)

There were loud cheers from the Margaret Court Arena crowd when Cristian held serve in the second set to avoid the dreaded 'double bagel'.

"It definitely gives me a lot of confidence winning matches like that, knowing I'm playing really focused every single point and not letting any kind of points or games go to waste," said Pegula.

"Definitely first matches are always really tough, especially at a slam, there's so much hype and anxious nerves leading up. So I'm glad it just went very smooth."

Danielle Collins, runner up to Ash Barty at Melbourne Park last year, had an injury timeout for a knee issue before grinding out a 7-5 5-7 6-4 win over Anna Kalinskaya.

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Danielle Collins of the United States hits a return against Russia's Anna Kalinskaya during their women's singles match on day one of the Australian Open tennis tournament in Melbourne on Jan 16, 2023. (MANAN VATSYAYANA / AFP)

Jessica Pegula hit her stride early in the new season by helping the United States win the inaugural United Cup, and the 28-year-old simply overpowered her Romanian opponent on Monday

"Any time you're on the court playing three-hour matches, you especially want to win, so I just needed to push through the hurdle a little bit," the 13th seeded American said.

"Definitely had a scare there with the injury in the first set but I was able to work through it. I'm just counting my lucky stars there a bit, it really was not a good start for me."

Convincing wins

Amanda Anisimova was unable to continue the good start for American women at the first Grand Slam since Serena Williams hung up her racket, the 28th seed tumbling out 6-3 6-4 at the hands of Ukrainian Marta Kostyuk.

With Barty and Williams both retired, and twice champion Naomi Osaka pregnant with her first child, Melbourne Park will almost certainly crown a first-time women's champion this year.

ALSO READ: Djokovic, Swiatek begins Australian Open as big favorites

Emma Raducanu of Britain plays a forehand return to Tamara Korpatsch of Germany during their first round match at the Australian Open tennis championship in Melbourne, Australia on Jan 16, 2023. (NG HAN GUAN / AP)

There were convincing early wins on Monday for two former US Open champions who have been unable to build on their Grand Slam success.

Canadian Bianca Andreescu, the 2020 champion at Flushing Meadows, beat Marie Bouzkova 6-2 6-4, while 2021 winner Emma Raducanu made light work of Tamara Korpatsch in a 6-3 6-2 victory.

Briton Raducanu, now 20, will next face teenager Gauff, who came into the year's first Grand Slam on the back of a title win in Auckland and confirmed her good form with a 6-1 6-4 win over Katerina Siniakova on Rod Laver Arena.

Greek sixth seed Maria Sakkari also had a straightforward 6-1 6-4 win over China's Yuan Yue to set up a tie against Russian qualifier Diana Shnaider.

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Italian 15th seed Jannik Sinner, a quarter-finalist last year, was the first man to reach the second round, rolling over Briton Kyle Edmund 6-4 6-0 6-2 on John Cain Arena.