Petra Kvitova rallied to defeat Venus Williams 3-6, 6-3, 7-6 (2) on Friday and advance to the Pan Pacific Open final.
The seventh-seeded Kvitova dominated the tiebreak, jumping out to a 6-0 lead.
Williams won the next two points before the Czech player capitalized on her third match point.
Williams had been bidding for her first title since winning in Luxembourg last October.
"I was probably a little tired," said Williams, who endured a three-hour, three-setter on Thursday.
"But I don't think it affected the match. She played two matches yesterday so no doubt was a little tired as well."
Kvitova, who held off 11 break points, will face either fourth-seeded Caroline Wozniacki or No. 5 Angelique Kerber.
"I am really happy with how I played today," Kvitova said. "It was a very close match and every game was up and down."
Serena Williams is world No. 1
The WTA has announced that Serena Williams is guaranteed to finish the year as the world No. 1 player for the third time.
This season, she has lost only four singles so far.
Williams, who also finished the year on top in 2002 and 2009, has won nine WTA titles this season including the French Open and U.S. Open.
The 32-year-old Williams returned to the No. 1 spot in February, when she became the oldest player to top the rankings.
She is the seventh player to finish at least three seasons at No. 1, alongside Steffi Graf (eight times), Martina Navratilova (seven times), Chris Evert (five times), Lindsay Davenport (four times), Justine Henin (three times) and Martina Hingis (three times).