Martina Navratilova and Chris Evert

Sports rivalries aren't always defined by wins, losses and hostility. OK, maybe they are when it comes to Duke and Carolina, the Yankees and Red Sox, and Ohio State and Michigan. But the story of Martina Navratilova and Chris Evert is as much about friendship as it is about Grand Slam titles. The duo waged a tennis tug of war that dominated their sport for much of the 1970s and '80s, but they also shared an off-court bond that rarely received any media attention. Unmatched, this week's entry in ESPN's 30 For 30 documentary series, uses this surprisingly warm friendship as a unique window into their rivalry.

The film is essentially a conversation between the two women, filmed over three days in Amagansett, Long Island, last October. With obvious love and respect, they reminisce about what they had in common — and what they did not. "I couldn't understand how you could just cry on the court," Evert says to Navratilova, who was known for her emotional on-court outbursts, especially early in her career. "But I would look at you with mixed emotions about it... 'Gosh, why can't she control herself? We're in front of 60 million people on TV.' But the other side of me really secretly admired that you could just let your emotions out and you didn't have any boundary in just showing everybody how you felt."

They played a staggering 80 times (with Navratilova holding a 43-37 edge in the series), but aside for a period of time when Navratilova's relationship with basketball star Nancy Lieberman put a strain on their friendship (Lieberman insisted that Navratilova had to hate Evert in order to beat her), they remained close. In Unmatched they talk about how they would even practice together just hours before facing each other in tournaments, including the French Open finals in 1974 and '75. And they briefly played as a doubles team, winning Wimbledon in 1976. (The partnership fizzled, however, when Evert became frustrated by Navratilova's dominance in singles.) In 1986, when Navratilova returned to her native Czechoslovakia for the first time since defecting to the U.S., she reached out to Evert to join her on the American Federation Cup team, and the trip remains a cherished memory for both.

The press clearly favored Evert, but Navratilova never resented her for it and, in fact, long before Navratilova gained widespread public support, Evert stuck by her friend. Navratilova especially appreciated Evert's support when she came out of the closet. "Even before it was OK to be gay friendly she was gay friendly," Navratilova says.

The format of the documentary, especially when Navratilova and Evert are driving a car and chatting, can be a bit jarring. But directors Lisa Lax and Nancy Stern Winters — veterans of NBC's Olympic coverage and ABC's Wide World of Sports, respectively — have constructed a definitive look at this iconic relationship.

Unmatched airs Tuesday at 8/7c on ESPN.

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