World number one Lindsay Davenport began her quest for a record fifth Pan Pacific Open title with a comfortable 6-4, 6-3 win over Japan's Saori Obata on Thursday.
Russia's Elena Dementieva crushed Japan's Akiko Morigami 6-2, 6-2 to join Davenport in Friday's quarter-finals.
Defending champion Davenport, beaten by Serena Williams in the Australian Open final last weekend, dropped her first two service games in Tokyo.
The American uncharacteristically threw down her racket in disgust after squandering five set points at 5-3 in the first set.
But she quickly recovered and went on to clinch victory with a pummelling cross-court backhand to reach the last eight of the $1.3 million event she won in 1998, 2001, 2003 and 2004.
"To be honest, I still felt a little worn down," said Davenport, who suffered a final set collapse in Melbourne in her 2-6, 6-3, 6-0 defeat by Williams.
"A Grand Slam final is the toughest loss to get over. I probably had the biggest lapse of my career and it's going to take a few weeks to get it out of my system."
Davenport next plays Czech Iveta Benesova, who battled past Japan's Aiko Nakamura 3-6, 7-5, 6-4.
Fourth seed Dementieva had few problems against Morigami and will next face another Japanese player, Shinobu Asagoe, who overwhelmed Italy's Roberta Vinci 6-0, 6-2.
Dementieva lost two Grand Slam finals last year Anastasia Myskina in French Open and Svetlana Kuznetsova at the U.S. Open, but believes she could go one better this season.
"I feel much stronger now and I know what it takes to reach a Grand Slam final," said Dementieva. "I just feel I need to take one more step and I think I can do that this year."
Sixth seed Elena Likhovtseva battled past American Jill Craybas 7-5, 4-6, 6-2 to set up an all-Russian quarter-final with Wimbledon champion Maria Sharapova.
Slovakian eighth seed Daniela Hantuchova beat Russia's Maria Kirilenko 4-6, 6-0, 6-2 and will play U.S. Open champion Kuznetsova in the last eight.
(Additional reporting by Ben Rimron)
