World number one Serena Williams was in dominant mood as she overpowered Australian Nicole Pratt 6-1 6-0 to reach the semi-final of the Toyota Princess Cup.

Defending champion Jelena Dokic also booked her place in the last four after beating Tamarine Tanasugarn of Thailand 6-3 6-1 in Tokyo's Ariake Colosseum.

Williams, aiming for back-to-back titles after beating sister Venus in the US Open final, fired down seven aces and a succession of stinging returns to close out the match in 39 minutes.

She will meet compatriot Amy Frazier in Saturday's semi-final.

"I'm starting to get focused. I'm almost where I want to be for the first tournament after the US Open," said Williams, who has won six times this year, including the last three grand slams.

"It's really hard to get re-focused after such a big win but I'm getting there."

Dokic, chasing her third title of the year, raced out to a 3-0 lead against seventh seed Tanasugarn before eventually taking the opening set 6-3.

Dokic raced on to victory in 64 minutes to book a date with third seed Kim Clijsters.

The Belgian overpowered Elena Likhovtseva of Russia 6-2 6-2 in the day's last singles match.

"I lost to (Tanasugarn) in the Hopman Cup two years ago but today I felt comfortable and I was dictating most of the points," said Dokic.

The 19-year-old Yugoslav has played most tournaments unaccompanied by father and coach Damir Dokic over the past 18 months.

"I've grown up a lot. Winning here last year was a big experience and gave me a lot of confidence," she said.

"I'm more independent now and that has helped me deal with things on and off the court."

Frazier recovered from a shaky start to beat sixth seed Tatiana Panova of Russia 2-6 6-1 6-4 in the day's first quarter-final.

Friday's results from the Toyota Princess Cup:
Quarter-final:
Amy Frazier (US) bt Tatiana Oanova (Rus) 2-6 6-1 6-4
Kim Clijsters (3) (Bel) bt Elena Likhovtseva (Rus) 6-2 6-2
S Williams (1) (US) bt Nicole Pratt (Aus) 6-1 6-0
J Dokic (2) (Yug) bt Tamarine Tanasugarn (7) (Tha) 6-3 6-1