Defending Australian Open men's champion Jannik Sinner and women's second seed Iga Swiatek moved into the second round with straight sets victories on Monday amid fierce attention over their doping sagas.

Four-time Grand Slam champion Carlos Alcaraz, 10-time tournament winner Novak Djokovic and women's third seed Coco Gauff also launched their campaigns with opening round wins.

Carlos Alcaraz hits a return during the men's singles first round match between Carlos Alcaraz of Spain and Alexander Shevchenko of Kazakhstan at Australian Open tennis tournament in Melbourne, Australia, Jan. 13, 2025. (Photo by Chu Chen/Xinhua)

It was a bumper day two schedule at a sunny and warm Melbourne Park in contrasting conditions to yesterday's stormy weather.

Top seed Sinner overcame big-serving Nicholas Jarry 7-6 (2), 7-6 (5), 6-1 in a physical first round match at Rod Laver Arena.

Sinner was unable to crack the towering Chilean's serve until early in the third set, having earlier relied on his composure to prevail in the tiebreaks.

The 34th ranked Jarry, who made the fourth round of the 2023 French Open, fired down 13 aces and hit 40 winners, but had 38 more unforced errors than Sinner.

It was a relief for Sinner, who has been shrouded under a doping cloud after he twice tested positive last year for traces of the steroid clostebol.

"I'm very, very happy to be back here," said Sinner, who beat Daniil Medvedev in last year's Australia Open final. "Today was a very close one. When I broke for the first time in the third set, it gave me room to breathe."

Alcaraz started his bid for a first Australian Open title in impressive style with a 6-1, 7-5, 6-1 thrashing of Kazakhstan's Alexander Shevchenko.

"There is no secret, I work, I try to be better every day," said 21-year-old Alcaraz, who is striving to win the only Grand Slam title that has eluded him.

Djokovic overcame a slow start to record a 4-6, 6-3, 6-4, 6-2 victory over teenage American wildcard Nishesh Basavareddy.

With new coach Andy Murray watching on, seventh seed Djokovic was left stunned after losing the first set before taking control to reach the second round in Melbourne for an 18th consecutive year.

"These kinds of matchups are always tricky, dangerous when you are playing someone who has nothing to lose," said Djokovic, who is bidding for a record 25th Grand Slam title.

Former finalist and 11th seed Stefanos Tsitsipas became the first big casualty of the tournament after being stunned by 42nd ranked Alex Michelsen, who clinched the biggest victory of his career 7-5, 6-3, 2-6, 6-4.

The 20-year-old Michelsen proved why he is highly-touted after blasting past a sluggish Tsitsipas, who has been off the boil in recent times having just two years ago lost to Djokovic in the Australian Open final.

Rising Chinese tennis star Shang Juncheng retired hurt in his first round match against Spain's Alejandro Davidovich Fokina.

Having lost the first set in a tiebreak, Shang had a medical timeout after the fifth game of the second set. He returned but moved gingerly before retiring hurt with Davidovich Fokina leading 7-6 (1), 5-2 in a match that lasted an hour and 51 minutes.

Former Wimbledon finalist Nick Kyrgios crashed to a straight sets defeat to world No.86 Jacob Fearnley in his first appearance at a major since the 2022 US Open.

In the women's, five-time Grand Slam champion Swiatek swatted away Katerina Siniakova, the world's number one doubles player, 6-3, 6-4 in an hour and 21 minutes.

Much like Sinner, Swiatek had entered the tournament under plenty of scrutiny having recently returned from her one-month suspension for testing positive to the banned substance trimetazidine.

Swiatek is hoping to brush that controversy aside as she bids for a maiden Australian Open title, with her best finish in Melbourne being a semi-final run in 2022.

"For sure, it wasn't an easy first round, so I'm happy I got through," Swiatek said. "I felt like she was playing really well and I knew I needed to be more proactive."

Swiatek will next play Rebecca Sramkova, who came back from a set down to beat American Katie Volynets.

Gauff, Swiatek's conqueror at the mixed teams United Cup final, continued to prove why she's a leading contender with a 6-3, 6-3 win in 80 minutes over former tournament champion Sofia Kenin.

Gauff won the season-ending WTA Finals last November before spearheading the United States' triumph at the United Cup.

"I knew going in it was going to be difficult, but I'm happy with how I played," she said of the win over Kenin, who has slid to No. 81 after being ranked fourth in 2020.

Four-time Grand Slam winner Naomi Osaka beat Caroline Garcia 6-3, 3-6, 6-3 to exact revenge after losing to the Frenchwoman in last year's first round in Melbourne.

It was a disappointment for China's Yuan Yue, who fell to 27th seed Anastasia Pavlyuchenkova in a three-set thriller.

Pavlyuchenkova held her nerve after a second set stumble to prevail 6-4, 4-6, 6-3 as the 43rd ranked Yuan crashed out of the tournament.

Medvedev will feature on Tuesday, along with fourth seed Taylor Fritz and former Wimbledon champion Elena Rybakina.

Zhang Zhizhen, China's top men's player, starts his campaign against 13th seed Holger Rune, while there will be five Chinese women in action on day three.