* Serena's first loss since opening round at French Open
* Venus edges U.S. Open champion Stosur in three sets (Adds Serena quotes)
By Simon Evans
MASON, Ohio, Aug 17 (Reuters) - Olympic champion Serena Williams suffered her first loss in nearly three months on Friday as she was knocked out of the Cincinnati Open quarter-finals, falling 6-4 6-4 to Germany's Angelique Kerber.
While older sister Venus Williams continued her progress and moved to the semi-finals with a 6-2 6-7 6-4 win over U.S. Open champion Samantha Stosur, Serena struggled against an impressive Kerber en route to just her fourth defeat of the year.
Serena, who had not lost since her opening round defeat at the French Open, complained of fatigue after her last 16 match on Thursday but she fell on Friday to an opponent who used a mix of intelligence and technique to make her life difficult.
"It was probably for the best that I didn't win today, even though I wanted to win. But maybe my body was like telling me not to. I don't know," said Williams, whose loss came in her 31st match, including singles and doubles action, since the start of Wimbledon.
"I felt really good going out there today, much better than my other two matches but it was a false alarm.
"She didn't do too much that threw me off. Her game is pretty straightforward and you know what to expect. She's getting a lot of balls back. She played really well today and I think she served more up the T than I expected."
Kerber, who has reached at least the quarter-finals in 12 of 17 events played this year, double-faulted on match point but on her third chance used a blistering ace to win the match.
"I had nothing to lose today, I tried to play my best and take it point by point because she has been playing so well recently," said world number seven Kerber.
The German's opponent in Saturday's semi-final will be the winner of the match between Russian Anastasia Pavlyuchenkova and Czech Petra Kvitova later on Friday.
Venus Williams's recovery from health problems is finally delivering some results, she reached her first semi-final since pulling out of last year's U.S. Open with an auto-immune disease that causes fatigue and joint soreness.
"I'd like to think that this is only the beginning for me of more semi-finals. Of course I'm going to stay focused and try to make it to the final as well," said Williams, who will face either China's Li Na or top seed Agnieszka Radwanska of Poland in the semi-finals.
"I'd like to take it a lot further. I haven't played a lot since 2010, so this is awesome for me already." (Editing by Frank Pingue)