* Li ends Venus run
* Williams suffers more injury woe (Adds quotes, details)
By Simon Evans
MASON, Ohio, Aug 18 (Reuters) - China's Li Na defeated an injured Venus Williams 7-5 3-6 6-1 to reach the final of the Cincinnati Open on Saturday after an enthralling two-hour encounter.
Li will face the winner of the other semi-final between Czech Petra Kvitova and Germany's Angelique Kerber who were playing later on Saturday.
Williams, playing in her first semi-final since the 2010 U.S. Open, was limited in her serve speed which was hovering around a slow 75 mph mark after picking up a back strain shortly before the match begun.
"Just in my warmup it was bothering me. I was like 'Oh, no'. I tried to fix it between the warmup and the match, but it didn't work," Williams told reporters.
"Right now I don't know why my back hurts, I just know it hurts. After this I'm going to get an evaluation and see what exactly is happening," said Williams, who will be keen to regain full fitness for the U.S. Open later this month.
The American, who has comeback from illness this year, double-faulted on break point to go 3-2 down in the first set but, relying on her return game, she battled back to 5-5.
But another crucial double-fault on game point allowed Li to sneak back ahead 6-5 and then wrap up the set.
After receiving an on-court massage to her back, Williams roared back in the second set to win five consecutive games en route to taking it 6-3 to level the match.
But the exertion of her comeback cost her in the third set where she ran out of energy and was convincingly beaten 6-1 as Li went in for the kill.
Li, the ninth seed, is now a perfect 4-0 in her semi-final matches this year but has yet to win a final. Li's most recent loss coming at Montreal last week.
"It's amazing this year, because in the last two years I hardly got a point in North America. So I'm happy I can be in the final again," the 2011 French Open champion told reporters. (Reporting By Simon Evans. Editing by Patrick Johnston)