(CBS Local)– Tennis stars like Serena Williams and Roger Federer have been greatly impacted by the sport’s instant replay system Hawk-Eye.

Williams famously lost a point on a ball that was clearly in against Jennifer Capriati the year before the system was implemented at the 2004 U.S. Open and Federer had a call overturned by Hawk-Eye against Rafael Nadal in the 2009 Wimbledon final. Director Theo Anthony takes viewers behind the scenes of the replay system in his new ESPN 30 for 30 documentary short “Subject To Replay.”

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Anthony made sure to highlight that moment with Williams at the U.S. Open because it demonstrates how far the Hawk-Eye instant replay system has come.

“The Serena clip is important for a lot of reasons. One, you see how much she’s been up against for so long and two, she’s got this incredible mini denim skirt on,” said Anthony in an interview with CBS Local’s DJ Sixsmith. “She’s been doing her thing for so long and she’s such an icon. It shows the need for these technologies and no one wants to see injustice in the world. No one wants to see unfair calls and no one wants to see anyone screwed over like that. There is this very real need for an accuracy or measurement system.”

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The film goes all the way back to the 1800s to show how early photography served as a precursor to instant replay. While Hawk-Eye is one of the best replay systems in sports, it took a little while for the players and fans to get used to it.

“I really like to talk about the present through talking about the past,” said Anthony. “There’s a lot of moments I could’ve gone into with a super close call. I think the Federer clip is important for a number of reasons. You have something where your human eyes are saying this ball looked out, but the digital eyes say the ball is in. The way that Hawk-Eye is set up, it overrules human decision making. There’s no checks and balances. The Nadal clip is interesting because the data later revealed that the ball was called in by one millimeter.”

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“Subject To Review” premieres December 22.