The run of 16-year-old French wild card Tatiana Golovin was ended by American 25th seed Lisa Raymond.
Golovin had beaten seeds Anna Pistolesi and Lina Krasnoroutskaya on route to the fourth round but was no match for the 30-year-old American.
Raymond broke serve in Golovin's first service game and never looked back, breaking again before taking the set.
It was even easier in the second and she wrapped up her 6-2 6-0 win with a delicate cross-court volley.
Raymond, who will face Patty Schnyder in the quarter-finals, was delighted to have kept up her form after beating third seed Venus Williams in the previous round.
"It would have been pretty easy for me to have a let-down after playing so well against Venus but I just went out there and just kept playing the way I've been playing," she said.
"I don't think I've ever been able to put together four sets of tennis like the last two matches in my whole career.
"I've had one-offs, but not at this level and not at a Grand Slam. It feels great and hopefully I've got more great tennis left in me.
"I think it's down to a combination of hard work and belief."
Schnyder reached her first quarter-final down under with a 6-2 6-4 win over France's 29th seed Nathalie Dechy.
Dechy started well with an early break of serve but the Swiss, seeded 22, soon found her range and swept ahead.
It all seemed over when Schnyder went 5-2 up in the second set but Dechy fought back to 5-4.
However, there was no denying Schnyder and, with six breaks of serve in all, she completed the win in 73 minutes.