MASON, Ohio (AP) - Venus Williams' reinvigorating week ended with a bad back and gritty comeback that wasn't quite enough.
Williams played through back pain that forced her to get treatment and reduced her second serve to 63 mph by the third set. China's Li Na took advantage, reaching the Western & Southern Open final with a 7-5, 3-6, 6-1 victory Saturday night.
All week long, Williams showed she's learned how to cope with an immune system disease that causes tiredness and sore joints. She made it through three grueling, three-set matches to get to the semis, and was ready for another one.
The bad back was too much. By the end, she was just trying to get her serve over the net. She considered quitting, but decided to keep playing.
"I pretty much couldn't serve," she said. "But I wanted to try, especially since this is my first semifinal of the year. I wanted to try to see if I could get to the final. It didn't work out for me."
Li will be playing for her first title this season. She lost in finals at Sydney, Rome and Montreal, where Petra Kvitova beat her for the Rogers Cup title a week ago.
Kvitova couldn't turn it into a rematch. She got sloppy in the final set and lost to Angelique Kerber 6-1, 2-6, 6-4 in the other semifinal Saturday night. Kerber ended Serena Williams' 19-match winning streak Friday, keeping her in contention for her third title of the season.
Serena watched from a seat in the stands next to the players' tunnel while her older sister wore down. At one point in the final set, she yelled: "Break her!" Venus smiled, knowing she was in no shape to do it.
Her semifinal loss aside, it's been a kick-up-your-heels week for Williams, who faded last year because of injuries and the immune system disorder. She's on an upswing heading into the U.S. Open, which she had to miss last year because of the disorder.
She won an Olympic gold medal in doubles with her sister at Wimbledon, a moment she called the best of her career. On Cincinnati's hard courts, she reached her first semifinal of the season by winning one long match after another.
Li had the most challenging time of the women this week in Cincinnati. She had the last match on Thursday night, and it got postponed by rain. She ended up playing two matches a few hours apart on Friday, winning both of them to reach the semifinals.
Li beat Johanna Larsson 6-2, 6-2 in the opener Friday morning. Li took the court less than seven hours later and beat top-seeded Agnieszka Radwanska, who had a sore shoulder.
On Saturday, Williams was the one struggling. She said the back started to bother her during warm-ups and got worse as the match went along.
"Right now, I don't know why my back hurts," she said. "I just know it hurts. After this, I'm going to get an evaluation and see what exactly is happening."
Li noticed the speed on Williams' serve diminishing.
"I saw her serve and was like, `What's going on?"' Li said.
After the third game of the second set, Williams called for the trainer. She placed a white towel on the court and lay face-down for several minutes while the trainer worked on her lower back for several minutes, with Williams wincing.
"I thought the match to be over," Li said.
Not even close. Williams went back out on the court and broke Li's serve to even the set at 2-all.
Williams had trouble getting anything on her serve - she moved stiffly - and ambled to her chair during breaks, leaning down to stretch her back. With the crowd cheering every point, she kept playing and kept winning, breaking Li again to go up 4-2 and serving out the set.
Before one serve early in the third set, she started to raise her arm to toss the ball and had to stop because of her back. She then served at 66 mph - and held serve. One of her second serves clocked 63 mph a few games later.
Williams pushed herself to the end. She ran down a ball for a put-away slam in the last game, which Li won when Williams hit a return long - her 47th unforced error during the 2-hour, 8-minute match.
Williams smiled as he walked to the net to shake hands.
"I like to live life with no regrets," she said. "I don't want look back and feel like I gave up or say I could have done this or that. That's not me. I wanted to go the very end and at least know that maybe I missed some shots or maybe I wasn't feeling my best, but I gave it my all."
Kvitova led the final set of her match slip away. Her double fault helped Kerber break to go ahead 3-2. The Czech had 15 unforced errors in the first six games of the set.
She avoided a match point in the ninth game, but Kerber served it out.