MELBOURNE, Australia -- Americans Jessica Pegula, Coco Gauff and Danielle Collins reached the second round in contrasting fashion at the Australian Open on Monday.
Third-seeded Pegula overwhelmed Jaqueline Cristian of Romania 6-0, 6-1 while seventh-seeded Gauff overcame a second-set wobble to beat Katerina Siniakova 6-1, 6-4.
Collins, the runner-up last year to Ashleigh Barty and seeded 13th, battled a left knee injury while defeating Anna Kalinskaya 7-5, 5-7, 6-4.
Pegula and Gauff could meet in the semifinals while Collins could play No. 1 Iga Swiatek in the fourth round.
A quarterfinalist in Melbourne in each of the past two years, Pegula needed just 59 minutes to get past the 161st-ranked Cristian, who was appearing in her third Grand Slam event.
"Today is just one of those days everything was working," said the American, who will play either Czech qualifier Brenda Fruhvirtova or Aliaksandra Sasnovich in the second round.
"Always feels good when you win a match like that. I think when those days come you just kind of take it and don't complain and don't critique. You kind of just move on to the next one."
An "extremely disappointed" Nick Kyrgios pulled out of the Open -- a day before he was scheduled to play his first-round singles match -- because of an injured knee that needs arthroscopic surgery.
Kyrgios, a 27-year-old from Australia, was the runner-up to Novak Djokovic at Wimbledon last year in singles and teamed with good friend Thanasi Kokkinakis for the men's doubles championship at the 2022 Australian Open.
Kyrgios was considered the host country's strongest chance to win a title at Melbourne Park this year; no man from Australia has won the singles trophy there since 1976.
He announced his withdrawal on Day 1 of action at the year's first Grand Slam tournament, appearing at a news conference at Melbourne Park alongside his physical therapist, Will Maher.
"I'm just exhausted from everything. Obviously pretty brutal," Kyrgios said of the decision to sit out. "One of the most important tournaments of my career. Hasn't been easy at all."
He was seeded 19th in Melbourne and was supposed to face Roman Safiullin in the first round on Tuesday.
Gauff, who reached her first Grand Slam singles final at the French Open last year, was given the honor of opening proceedings on Rod Laver Arena and dominated Siniakova in the first set.
The Czech player led 4-2 in the second set but Gauff rallied and clinched victory on her seventh match point.
"I was not expecting to open the tournament on Rod Laver," Gauff said. "I'm super-honored that the tournament chose me and Katerina. I'm really pleased with myself. Katerina's a fighter, I knew she was going to fight for every point. I just stayed strong mentally."
Gauff will now play former U.S. Open champion Emma Raducanu after the British player shrugged off a bothersome left ankle issue to beat Tamara Korpatsch of Germany 6-3, 6-2.
Collins required a medical timeout for a left knee issue early in the first set of her match with Kalinskaya but survived to win in just over three hours.
Another former U.S. Open champion, Bianca Andreescu, advanced to the second round but 28th-seeded Amanda Anisimova was beaten 6-3, 6-4 by Marta Kostyuk.
Danielle Collins of the U.S. plays a forehand return to Anna Kalinskaya of Russia during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Dita Alangkara)
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, Monday, Jan. 16, 2023. (AP Photo/Ng Han Guan)
Bianca Andreescu of Canada reacts after winning a point against Marie Bouzkova of the Czech Republic during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Ng Han Guan)
Katerina Siniakova of the Czech Republic plays a forehand return to Coco Gauff of the U.S. during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Aaron Favila)
Jaqueline Cristian of Romania plays a forehand return to Jessica Pegula of the U.S. during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Dita Alangkara)
Coco Gauff of the U.S. celebrates after defeating Katerina Siniakova of the Czech Republic in their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Aaron Favila)
Jessica Pegula of the U.S. plays a backhand return to Jaqueline Cristian of Romania during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Dita Alangkara)
Jessica Pegula of the U.S. plays a backhand return to Jaqueline Cristian of Romania during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Dita Alangkara)
Jessica Pegula of the U.S. plays a forehand return to Jaqueline Cristian of Romania during their first round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 16, 2023. (AP Photo/Dita Alangkara)