World No.6 Ruud traded sets with Spain's Jaume Munar before claiming a 6-3 1-6 7-5 2-6 6-1 victory on Sunday, while Japanese veteran Nishikori turned back the clock to record a stunning 4-6 6-7 (4-7) 7-5 6-2 6-3 win from match-points down over Brazilian world No.105 Thiago Monteiro.
With play suspended on outside courts as Melbourne Park copped a day-one drenching, a spotlight was cast on Ruud's seesawing encounter on Rod Laver Arena.
Kei Nishikori (pictured) saved two match points before beating Thiago Monteiro in five sets. (Joel Carrett/AAP PHOTOS)
The sixth-seeded Norwegian left the court for treatment after losing the fourth set, but came out firing in the fifth, breaking Munar twice to take out the decider.
Ruud admitted to the world No.62 Spaniard that he got a bit lucky to book a spot in the second round, where he will face the winner of the clash between Czech teen Jakub Mensik and qualifier Nikoloz Basilashvili.
"It was a really tough match," Ruud said after the encounter, which lasted three hours and 21 minutes.
"I know Jaume well because we are both practising a lot in the Rafael Nadal Academy in Majorca.
"So when you know someone quite well it's always tricky because he knows my game, I know his game."
Ruud, a two-time French Open runner-up who also reached the 2022 US Open final, has never been past the fourth round at Melbourne Park - a feat equalled by his father and coach Christian.
"I have a better record in most of the other tournaments," he said.
"This is kind of the last one standing for him. I hope I can beat it one day, but I'm happy if he also has one record at least in the family."
On John Cain Arena, former US Open finalist Nishikori saved two match points in the third set, turning the tide on a tiring Monteiro to triumph in four hours and six minutes.
It is the fifth time in his career Nishikori has come back from two sets down to win.
The 35-year-old has not appeared at Melbourne Park since 2021 after missing the entire 2022 season and playing only four tournaments in 2023 due to a series of injuries.
His ranking plummeted to as low as No.759, but he has scrapped his way back to world No.74.
Nishikori will next face either American world No.12 or local hope Chris O'Connell.
Earlier, rising French star Arthur Fils became the first man through to the second round.
Arthur Fils took care of business early on day one at Melbourne Park. (James Ross/AAP PHOTOS)
Playing under the closed roof on Margaret Court Arena, No.20 seed Fils overcame a slow start to beat Finland's Otto Virtanen 3-6 7-6 (7-4) 6-4 6-2 in two hours and 35 minutes.
The 20-year-old enjoyed a breakout season in 2024, winning titles in Hamburg and Tokyo.
He also recorded his best result at a major, advancing to the fourth round at Wimbledon before bowing out to Australia's Alex de Minaur.
That was the match when de Minaur suffered the hip injury on match point which forced him to pull out of his much-anticipated quarter-final against Novak Djokovic and impacted the rest of his season.
In other day-one results, second seed Alexander Zverev cruised to a 6-4 6-4 6-4 victory over Lucas Pouille, while 24th seed Jiri Lehecka downed Australian wildcard Li Tu 6-1 3-6 6-3 7-6 (7-1).
Fellow Czech Tomas Machac had a straight sets win over India's Sumit Nagal, Ugo Humbert, the 14th seed, beat Italian qualifier Matteo Gigante 7-6 (7-5) 7-5 6-4 and American Reilly Opelka, finalist in Brisbane, scrapped to a four-set win over Belgian qualifier Gauthier Onclin.
