Clijsters and Serena to head Tokyo field
The two former world number ones and multiple Grand Slam winners will be making their first appearances at the $2 million (£1.2m) dollar event, which begins on September 25.
Belgian Clijsters, who won the year's first Grand Slam at the Australian Open, is currently ranked second in the world and briefly held the top spot after capturing her fourth major in Melbourne in January.
Williams, who has won 13 Grand Slam singles title, has slipped to 11th in the rankings after being sidelined with a foot injury following her Wimbledon victory last season.
The American suffered a further setback when she underwent emergency treatment for a blood clot in one of her lungs earlier this week.
"We know about the surgery," the Pan Pacific Open's Atsuko Isoyama said. "But at this stage, Serena is formally among the players entered."
A total of six top 10 women's players are scheduled to compete in this year's Pan Pacific Open.
Japanese veteran Kimiko Date-Krumm, who won the competition in 1995 before she left the game for 12 years, will look to strike another blow for 40-somethings in Tokyo.
Last year, the former top five player shocked former world number one Maria Sharapova in the first round before her dream run was ended in the third round by Schiavone.