Grigor Dimitrov was pushed all the way in five sets before finally beating No. 186-ranked Mackenzie McDonald, who had never won a tour-level match before qualifying for the Australian Open.
Dimitrov won the season-ending ATP Finals last November and entered the tournament ranked No 3, but struggled in a second-round night match which finished 4-6, 6-2, 6-4, 0-6, 8-6 as midnight approached Wednesday.
McDonald, the NCAA champion for UClA in 2016, had never played anyone ranked in the top 50 but was far from overwhelmed.
He attacked Dimitrov's backhand, turning a potential weapon into a liability for the Bulgarian, and hia second serves.
McDonald had three service breaks in the fourth set to force a deciding fifth and pushed it beyond 12 games — there's no tiebreakers in the fifth sets at the Australian Open — before his first double-fault of the set gave Dimitrov a match point.
He converted it on the next point when McDonald buried a forehand into the net.
"It's a tough day, obviously," Dimitrov said. "He played an unbelievable game. There's not much I can say - experience in the end really helped me."
That seemed to be a theme of Day 3.
2008 runner-up Jo-Wilfried Tsonga had to come back from 5-2 down in the fifth for his win.
Rafael Nadal, the 2017 runner-up, wasted no time in reaching the third round, dropping only one service game — while serving for the match — and making just 10 unforced errors in a 6-3, 6-4, 7-6 (4) win over leonardo Mayer.
"It's an important victory for me, I mean, he's a tough opponent. leonardo is a player with big potential," said Nadal, who won the French and US Opens last year but had his preparation for Australia delayed because of an injured right knee. "After a while without being on the competition ... second victory in a row, that's very important."
Tsonga rallied from 5-2 in the fifth to overcome Denis Shapovalov 3-6, 6-3, 1-6, 7-6 (4), 7-5 in a 3-hour, 37-minute match that contained one of his nonchalant between-the-legs shots on an important point.
In what shapes as an entertaining showdown, he'll meet No 17-seeded Nick Kyrgios in the next round.
Kyrgios had a 7-5, 6-4, 7-6 (2) win over Viktor Troicki, overcoming audio problems at Hisense Arena and complaining to chair umpire James Keothavong, who ended up turning off his microphone and later being hit in the head by a wayward tennis ball.
And 38-year-old Ivo Karlovic overcame Yuichi Sugita 7-6 (3), 6-7 (3), 7-5, 4-6, 12-10.
Dimitrov will next play No 30 Andrey Rublev, who beat Dimitrov at the US Open and who accounted for 2006 Australian Open finalist Marcos Baghdatis 6-4, 6-7 (5), 6-4, 6-2.
At the other end of the experience spectrum, 15-year-old qualifier Marta Kostyuk followed up her first-round win over 25th-seeded Peng Shuai with a 6-3, 7-5 victory over wild-card entry Olivia Rogowska.
The Australian Open junior champion last year, who entered the season-opening major ranked No 521, Kostyuk became the youngest player since Martina Hingis in 1996 to win main draw matches at the season-opening major.
Things will get harder for her now, against fellow Ukrainian and No 4-seeded Elina Svitolina, who had a 4-6, 6-2, 6-1 win over Katerina Siniakova.
Another Ukrainian, Kateryna Bondarenko, beat No. 15-seeded Anastasia Pavlyuchenkova 6-2, 6-3 and will next play No. 19 Magdalena Rybarikova.
Belinda Bencic had a letdown two days after upsetting Venus Williams, losing 6-1, 6-3 to Thai qualifier luksika Kumkhum.
Bencic, who combined with Roger Federer to win the Hopman Cup for Switzerland earlier this month, saved three match points on her serve before netting a backhand to give No. 124th-ranked Kumkhum a spot in the third round for the first time.
"I tried to reset and focus on the next match," Bencic said. "I think it was also a very tough second round, for me the toughest I could get."
Alize Cornet beat No 12-seeded Julia Goerges 6-4, 6-3, knocking the sixth of eight seeds out of that quarter of the draw.
No 42-ranked Cornet had taken only one set in five previous losses to Goerges, who was on a 15-match winning streak after her opening win at Melbourne Park.
French Open winner Jelena Ostapenko struggled at times before beating Duan Yingying 6-3, 3-6, 6-4.
Among the seeded men advancing were No 6 Marin Cilic, who beat Joao Sousa 6-1, 7-5, 6-2, and No 10 Pablo Carreno Busta, No 23 Gilles Muller and No 28 Damir Dzumhur.
Ryan Harrison beat No. 31 Pablo Cuevas 6-4, 7-6 (5), 6-4 and Kyle Edmund had a straight-sets win over Denis Istomin — who beat then defending-champion Novak Djokovic in the second round here last year.