World No.1 Serena Williams has pulled out of next week’s Pan Pacific Open tennis tournament in Tokyo over fitness concerns.

The 31-year-old American will skip the September 22-28 tournament because she is “in bad condition”, the tournament office said.

Russia’s Maria Sharapova, the world No.3, said last week that she would miss the $US2.4 million ($A2.55 million) tournament to recover from a right shoulder injury.

“Unfortunately, with the number of matches I have played throughout the summer, I need more time to properly recover,” Williams was quoted as saying by Kyodo news agency.

Her official website shows she is still scheduled to play at the China Open in Beijing the week after the Pan Pacific event.