Top seed Jelena Jankovic blazed into the quarter-finals of the Pan Pacific Open with a crushing 6-2, 6-1 victory over Italian Flavia Pennetta on Thursday.
The Serb, beaten by Serena Williams in the US Open final earlier this month, will recapture the world number one ranking from the American if she wins the Tokyo title.
Earlier, Russia's Olympic champion Elena Dementieva booked her place in the season-ending WTA Championships with a 6-0, 6-3 win over France's Alize Cornet in her opening match.
Jankovic overpowered Pennetta in a match played under a closed roof because of an approaching typhoon, wrapping up an easy win with an ace after just 61 minutes.
"I would like to be number one again," Jankovic told reporters. "But the most important thing is to finish the year as the number one player."
Jankovic faces Russian Svetlana Kuznetsova next after the fifth seed blew away Japan's Ayumi Morita 6-1, 6-1.
World number four Dementieva, who won gold in Beijing last month, took just 69 minutes to reach the last eight of the $1.34 million (735,000 pounds) Pan Pacific tournament.
The third seed became the fifth player to qualify for November's showcase event in Doha, exclusive to the world's top eight. "It's great to be there months before," said Dementieva, winner in Tokyo two years ago. "I remember years before fighting right up to the end to get into the tournament."
World number one Serena Williams, Serbs Jankovic and Ana Ivanovic and Russia's Dinara Safina have already clinched their spots in the $4.45 million Doha shootout.
Six of the world's top 10 players are appearing in Tokyo, although number one Williams is not taking part.