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Susan Mullane-USA TODAY Sports

It was a good year for tennis stars. Famous names such as Rafael Nadal, Novak Djokovic and Serena Williams ruled the 2013 tennis season.

But that doesn't mean it was a straightforward year. Far from it. Marion Bartoli won one of the wackiest Wimbledons ever, precocious teenagers on the WTA tour defeated Grand Slam Champions and Roger Federer found himself confronting his humanity time and time again.

In other words, there were definitely some memorable upsets. Let's take a look at the eight upsets that kept us on our toes in 2013.