China's Li Na, seeking her first WTA title since winning the 2011 French Open, defeated Venus Williams 7-5, 3-6, 6-1 on Saturday to reach the final of the ATP and WTA Cincinnati Masters.
Ninth seed Li, who reached the Montreal final last week, will play Sunday's final against the winner of a later match between fifth seed Angelique Kerber of Germany or Czech fourth seed Petra Kvitova, the 2011 Wimbledon winner.
Li captured the first set and broke Williams' first service game of the second set for a 2-0 edge when the American, nagged by back pain, called for the trainer, receiving a back rub and returning to the court.
The treatment made all the difference to former World No. 1 Williams, who won the next five games on her way to forcing a third set that Li dominated, improving to 3-0 all-time against Williams with her 33rd victory of the season.
"I'm sorry for Venus. She had a back problem and she really couldn't serve. But after she had treatment she really fought hard in the second set," Li said.
"When she was stretched out on the court I thought maybe the match might be finished. But there are no easy matches on this tour and it went all the way."
Williams produced two of her eight double-faults on break points to lose games.
Li, beaten by Kvitova last weekend for the Montreal title, has lost three finals in all this year.
But Li, 30, is playing her first event with new coach Carlos Rodriguez, a former mentor to seven-time grand slam winner Justine Henin who currently coaches at an academy in Beijing.
Rodrigeuz got the call only days ago from Li's agent after the player decided to cut loose her husband as her coach and go with a professional.
"I had a losing experience last week," Li said. "I hope to do better here. Maybe a new coach can help in that. I know he did a good job with Henin and I'm sure we can work together well."