The 31-year-old South African produced another typical never-say-die performance to haul his way back from the brink of defeat and earn a spot in the last 16.
"After the first two sets, I didn't think I had a chance," Ferreira said.
"He was beating me fair and square but then he gave me the third set.
"By the fourth I was getting close and then I felt like I was the better player."
Ferreira made the quarter-finals of last year's Australian Open after surviving two epic five setters.
The former world number six fought back from two sets and 1-5 down in the third to beat Croatian Ivan Ljubicic in the third round then defeated Albert Costa 9-7 in the fifth set of their fourth round encounter.
Despite being 10 years older than 21-year-old Fish, Ferreira said he could see his younger opponent starting to wilt in the heat the longer the contest went.
"It's great, it's absolutely fantastic (to see an opponent struggling)," Ferreira said. "Its the best joy you can get actually."
Fish, who knocked out former world number one Carlos Moya in a five-set second round match, said he could not believe he had lost. The American broke to lead 4-2 in the fourth set but did not win another game as Ferreira stormed to victory.
"I really don't know what happened but I think experience played a major part of it," he said.
"I lost the last 10 games in a row and that's not acceptable at this level.
"But hopefully this experience will help my career and hopefully when I'm in that position again I can take advantage."
Americans Andre Agassi and Venus Williams safely made it through to the last 16 after recording contrasting wins.
Agassi had to dig deep to overcome determined Frenchman Nicolas Escude 6-2, 3-6, 6-3, 6-4 while Williams crushed Germany's Anca Barna 6-1, 6-4.
The only big casualty of the day was French Open champion Albert Costa. The eighth seed blew a two sets to one lead in losing to fellow Spaniard Felix Mantilla 3-6, 6-3, 4-6, 6-1, 6-3.
Second seed Agassi stayed on course to become the first foreign player to win four Australian Opens with a battling win over Escude in a see-saw match.
"I needed to play well, no question. I'm happy to be through that," a relieved Agassi said.
"You never look ahead in the draw but he's a guy you never want to be near."
Williams's performance was littered with mistakes but the world number two raised her game whenever it mattered to complete an easy win over Barna.
Despite losing the last three grand slam finals and the number one ranking to her little sister Serena in 2002, Venus said she was starting to get back into the groove.
"I'm enjoying it a lot, especially when I'm winning," she said.
"When things get a little tight, I tend to be a little bitter about my performance, that's natural, but when I'm on a roll it definitely feels great."
POPULAR ATTRACTION
A popular attraction at the Open this year, the American will be public enemy number one when she faces Nicole Pratt in her next match.
Pratt upset 23rd seed Paola Suarez 7-5, 6-4 to become the first local women to reach the fourth round of the Australian Open in a decade.
"I've been a hard worker for a long time now...to be in the fourth round of a grand slam, especially the Australian Open, is very satisfying for me," the 29-year-old said after reaching the last 16 of a grand slam for the first time in her career.
"I have taken a few knocks, put in the hard work. I'm happy to be here."
Last year's Roland Garros runner-up, Spanish world number four Juan Carlos Ferrero had to fight back from a set down before he could book a place in the fourth round with a 4-6, 6-3, 4-6, 6-2, 7-5 win over Frenchman Fabrice Santoro.
Justine Henin-Hardenne, Lindsay Davenport and Daniela Hantuchova eased through the women's draw with straight set wins.
UNDERDOG
Davenport, calling herself an underdog here but playing like a hot favourite, will next play fifth seed Henin-Hardenne.
Davenport, Australian Open champion three years ago, beat Russian Tatiana Panova 6-2, 6-1 while the fifth-seeded Henin-Hardenne cruised past Slovenian Katarina Srebotnik 6-2, 6-0.
Agassi's good friend, Armenian Sargis Sargsian, made it past the third round of a grand slam for the first time with a 5-7, 7-5, 6-0, 6-4 win over Australia's Mark Philippoussis while Frenchman Sebastien Grosjean, Mario Ancic of Croatia and Argentina's Guillermo Coria also advanced.
Spain's Virginia Ruano Pascual ended German giantkiller Marlene Weingartner, who knocked out defending champion Jennifer Capriati in the first round, run in Melbourne with a 6-1, 4-6, 6-4 victory.
