Melbourne - Roger Federer said the new vulnerability of tennis's "top guys" was good for the sport as he avoided becoming yet another Australian Open casualty on Sunday.
On a day when world number one Andy Murray joined six-time winner Novak Djokovic on the sidelines, Federer advanced to the quarter-finals.
The Federer resurgence claimed the scalp of Japan's world number five Kei Nishikori over five gruelling sets and he will now play Murray's conqueror Mischa Zverev in the last eight.
Federer backed up his astonishingly easy win over Tomas Berdych with a 6-7 (4/7), 6-4, 6-1, 4-6, 6-3 victory over Nishikori that moves him closer to a surprise 18th Grand Slam title.
With no Murray and Djokovic in the draw, Federer's chances have immeasurably improved and given new belief to the others that they can go on and win the year's first major.
With Federer and his long-time Slam rival Rafael Nadal, the final member of tennis's former 'Big Four', rolling back the years, the Swiss maestro said the shock results have been good for the sport.
"I felt like if Rafa and myself can be healthy, yes, you can expect that (they would be in the quarter-finals). That Novak and Andy are not, that is a big surprise," he said.
"I never thought that Mischa Zverev and Denis Istomin (who beat Djokovic) would beat those two big guys.
"I guess it's good for tennis that a lot of guys believe strongly now that the top guys are beatable, are vulnerable, especially on a faster court."
Nishikori, who had lost his last three matches against Federer, was troubled by a left hip injury and twice needed courtside treatment from a trainer in Sunday's match.
"I felt great in the fifth set, I must say. Great energy," Federer said.
"Even deep into the fourth I thought, Yeah, fifth, here we go, no problem for me. I'm feeling good about my chances.
"Kei's a tough customer. This is a huge win for me in my career."
Federer holds a 2-0 career record over Zverev, who is into his first Grand Slam quarter-final after pulling off the victory of his career against Murray.
"Obviously he's on a high right now. He's feeling great. Probably feels the best he's ever felt on a tennis court," Federer said of Zverev.
"I like Mischa a lot and he has a nice game coming forward. It will be different."
Federer clawed back from a double break to take the opening set to a tiebreaker, only to lose it seven points to four.
The Swiss maestro skated through the second and third sets for the loss of five games and appeared set to round it off in four sets.
But Nishikori fought back hard and held a titanic, 10-minute service game in the fourth game before breaking Federer's serve in the next game.
He took the match to five sets, but he was troubled by the pre-existing hip injury and sought the trainer for treatment at two changeovers.
Federer broke in the second game of the final set and held the advantage through to the end to improve his career five-set record to 25-20.
The Swiss legend is looking to become the third man to win five Australian Open singles titles after Novak Djokovic and Roy Emerson, who have both won six.
It is the 13th time Federer has reached the last eight, extending his record for most Australian Open quarter-finals.