But there were no such wobble against the unseeded Marcos Giron on Rod Laver Arena as he sprinted home 6-3, 6-4, 6-2 in two hours and one minute, slamming 35 winners and eight aces.
Awaiting him next is Holger Rune after the Danish 13th seed defeated Serbian Miomir Kecmanovic in five sets.
“If I want to go on in this tournament, I have to improve,” said 23-year-old Sinner.
French showman Monfils rolled back the years to storm back and defeat the American Fritz 3-6, 7-5, 7-6 (7/1), 6-4. The unseeded Monfils, who hit a career-high ranking of sixth in 2016 but is now 41st, is enjoying a late-career flourish.
A week ago, he became the oldest singles champion in ATP Tour history when he swept to victory at the ASB Classic. Monfils faces another American in 21st seed Ben Shelton, who is a big fan.
“I think that any time he’s on the court, it’s a box-office match,” said Shelton.
Monfils is married to Svitolina, who emulated his giant-killing efforts when she defeated world No 4 Jasmine Paolini, also on Margaret Court Arena. Svitolina came though 2-6, 6-4, 6-0.
“I warmed up the court for her,” joked Monfils.
Tien became the youngest man to reach the fourth round since Rafael Nadal in 2005 when he overcame injured Frenchman Corentin Moutet.
The gifted teenager won 7-6 (12/10), 6-3, 6-3, with Moutet collapsing clutching his leg in the third set before gamely carrying on.
“Honestly, it feels pretty crazy to be in the second week,” he said.
Tien stunned three-time runner-up Daniil Medvedev in a five-set epic in the previous round.
Home hope Alex de Minaur, the eighth seed, went through in four sets against Argentina’s Francisco Cerundolo and will face American Alex Michelsen for a place in the quarter-finals.
In a one-sided battle of former US Open champions, Swiatek romped to a 6-1, 6-0 triumph in a statement victory over Raducanu as she pursues a first Melbourne crown.
Emma Navarro, the eighth seed from the United States, joined the Pole in the next round with a gritty three-set win over Ons Jabeur. Sixth seed Elena Rybakina also advanced despite a back problem.
“I felt like the ball is listening to me,” Swiatek said after rattling off 11 straight games in a brutal display against Britain’s Raducanu at Rod Laver Arena.
Twenty-three-year-old Swiatek is a five-time major winner but has never gone beyond the semifinals at Melbourne Park.
She faces world No 128 German Eva Lys next.
Lys beat Jaqueline Cristian of Romania in three sets to become the first women’s singles “lucky loser” to reach the fourth round since the event moved to Melbourne Park in 1988.
Former Wimbledon champion Rybakina of Kazakhstan needed a medical timeout to treat a back spasm before winning 6-3, 6-4 against Dayana Yastremska of Ukraine.
Asked if her back would be okay, she replied: “Not really. So I will see my physio, and hopefully he does some magic.”
Rybakina will face Madison Keys after she swatted aside fellow American Collins 6-4, 6-4, with Collins getting booed in and out of the arena.
Novak Djokovic, Carlos Alcaraz, Aryna Sabalenka and Coco Gauff are all in action on Sunday.
