Eighth-seeded Nishikori withstood 59 aces from Ivo Karlovic, and had to save three break points deep in the fifth, before beating the 39-year-old Croatian 6-3, 7-6 (6), 5-7, 5-7, 7-6 (7) to reach the third round.
After surviving a scare, Nishikori won the contest against Majchrzak 3-6, 6-7 (6), 6-0, 6-2, 3-0 in a match that lasted almost three hours. It's good to see him playing well.
In another super tight match, Milos Raonic took down a former champion Stan Wawrinka 6-7, 7-6, 7-6, 7-6 in four hours of titanic battle that has deserved to be at least the quarter-final clash. "I'll try to carry this confidence into the next match".
Nishikori will face Ivo Karlovic in the next round.
The 73rd-ranked Croatian slammed eight aces over his first four service games at Margaret Court Arena, but the 29-year-old Nishikori wrapped up the opening set in under 30 minutes, breaking his opponent's serve to take a 5-3 lead before closing the stanza with decisive service game topped with a forehand victor on set point.
But he let Majchrzak back into the game and when the set went to a tiebreaker, two unforced errors by Nishikori handed his opponent the set. Another hour after that, Wawrinka was two points from seizing the fourth to force a fifth but came up short.
Coming back strongly after two back to back defeats in the previous sets, Nishikori won 76 per cent of the points in the final set. It could have gone both ways. "Now I am nearly 40". "I didn't do anything, but he was hurt and I feel sorry for him".
"It's never easy, of course, and frustrating if you can't (return) serve three times in a row". "I think I focused well".
Chung has struggled to build on his breakout performance where he famously beat fourth seed Alexander Zverev and Djokovic en route to retiring in the second set against Federer with massive, deep blisters on both feet.
"Well, I have no idea how I won this tonight", said Halep, the reigning French Open champion.
Last year, Nishikori missed the season's first Gram Slam when a right wrist injury suffered in August 2017 wiped out the start of his season.
Nishikori star had to dig himself out of a 0-40 hole at 4-4 in the fifth set, and the match went to yet another tiebreaker from which Nishikori prevailed by winning the final four points.
The Japanese fans were still in place after the break to watch U.S. Open champion Naomi Osaka advance with a 6-2, 6-4 win over Tamara Zidansek.
"She's been playing really well recently so I just hope that it's exciting", she said.
The 21-year-old Osaka found herself trailing 4-2 in the next set, but turned the set around by breaking Zidansek twice.
This tournament marks Williams' 19th main draw appearance at the Australian Open to lead active players, and an open-era record 81st overall Grand Slam.
The fourth seed looked far from his anointed status as the flag-bearer for the next generation as he failed time and again to put away the veteran before winning 7-6 (7/5), 6-4, 5-7, 6-7 (6/8), 6-1.