Serena Williams Upset by Angelique Kerber at Western & Southern Open

Serena Williams, coming off a gold medal in the 2012 London Olympics, lost to world No. 7 Angelique Kerber in the quarterfinals of the Western & Southern Open on Friday (4-6, 4-6, per the Associated Press, via Yahoo! Sports).

She showed her frustration after losing a game in the second set, slamming her racket down on the ground.

Williams had won 19 straight matches this year leading up to the defeat, including a victory at Wimbledon. She was ranked No. 4 in the world coming into the tournament.

It must be noted that Kerber is no slouch. She had gone 48-14 this season headed into the Western & Southern Open, including two singles victories.

But it was still a letdown for Williams, who has won 14 Grand Slam singles titles in her career and had gone 2-0 against Kerber in her career, both wins coming in straight sets.

Still, it's a minor blip in what has been a fantastic season for Williams. After all, winning Wimbledon and a gold medal in the Olympics in the same year is nothing to scoff at. She was 43-5 with five singles titles this season headed into Cincinnati, Ohio.

Williams will regroup and give it another go at the 2012 U.S. Open beginning August 27. She's won the tournament three times in her career, most recently in 2008.

But she needs to keep her head. Any more of what we saw on Friday and she may be forced into an early exit at the U.S. Open, similar to the French Open this year when she was ousted in the first round by Virginie Razzano.