Li Na of China hits a return to Venus Williams of the US in their women's singles semi-final match at the Cincinnati Open tennis tournament in Cincinnati, Ohio August 18, 2012. [Photo/Xinhua]

MASON, Ohio - China's Li Na defeated a battling Venus Williams 7-5 3-6 6-1 to reach the final of the Cincinnati Open on Saturday after an enthralling two-hour nine minutes 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 US Open, looked to have a back problem and was limited in her serve speed which was hovering around a slow 75 mph mark.

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 wrapup 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 a perfect 5-0 in her semi-final matches this year but has yet to win a final.