Jannik Sinner says he does not yet "feel very safe" with the changes he has made to his serve after starting his Australian Open title defence with a shortened first-round win against Hugo Gaston.

The world number two and two-time defending Australian Open champion was in full control of the match at 6-2, 6-1 up when Frenchman Gaston abruptly retired.

Sinner led the ATP Tour in percentage of service games won in 2025 (92%), but feels there is still "a lot of room to improve".

Asked about changes he has made since his US Open final loss to Carlos Alcaraz in September, the 24-year-old Italian said: "I felt like the serve was and still is a shot where I need to improve.

"We changed the motion a little bit, the rhythm of the serve. Before, it was a bit too fast in the beginning; now it's a bit slower.

"At times, I still lose it. It's not a shot I feel very safe with, but we are working on that.

"It is one of, if not the most, important shots we have because it can give you so many great things."

Apart from the opening game of the match, when he recovered from 0-40 down to hold serve, Sinner was rarely troubled by world number 93 Gaston.

Playing his first official match since winning the ATP Tour Finals in November, Sinner sent down three aces and an unreturnable serve during a run of five consecutive points to avoid that early break.

He lost only nine more points behind his serve in the remainder of the match, served six aces in all and did not commit a double fault, ending with a first-serve percentage of 64%.

Four-time Grand Slam champion Sinner reeled off four consecutive breaks of serve as he won nine games in a row, moving from 2-2 in the first set to the brink of a two-set lead.

Gaston avoided a second-set bagel by holding serve at 5-0 down but the left-hander, who briefly received treatment at the end of the first set, elected to end the match after losing the second.

"Obviously it's not the way you want to win the match. I wish him a speedy recovery and hope it's nothing too bad," added Sinner, who will face Australian wildcard James Duckworth in round two.

'Thank you for this amazing ride' - Monfils says farewell to Australian Open

Gael Monfils' 20th and final Australian Open appearance ended in a first-round defeat by Australian qualifier Dane Sweeny.

The 39-year-old Frenchman, who will retire at the end of the season, was restricted physically in the latter part of the match but remained competitive, despite struggling to move freely.

Monfils went 4-1 up with a break in the fourth set but then lost six of the next seven games as Sweeny sealed a 6-7 (3-7) 7-5 6-4 7-5 win in three hours and 51 minutes.

"Somehow it's the finish line," Monfils told the crowd after his loss.

"Thank you so much for this amazing ride, you guys have been unbelievable.

"I've got a lot of great memories here, some big battles, even today was almost four hours.

"I've been very grateful and very lucky to play here for many years."

Elsewhere, Italian fifth seed Lorenzo Musetti led by two sets to one when his Belgian opponent Raphael Collignon retired injured in the fourth set.

Eighth seed Ben Shelton, a semi-finalist at Melbourne last year, won 6-3 7-6 (7-2) 7-6 (7-5) against Frenchman Ugo Humbert.

Fellow American Taylor Fritz, the ninth seed, former runner-up Stefanos Tsitsipas and 16th seed Jakub Mensik also made it through to the second round.

However, teenage Brazilian star Joao Fonseca appeared to be hampered by a back injury in a four-set loss to world number 85 Eliot Spizzirri of the United States, while Bulgaria's Grigor Dimitrov was another to make an early exit, beaten in 6-4 6-4 6-3 by Czech player Tomas Machac.