Rivalries can inspire epic matches and great debate.

Roger Federer has formed several compelling rivalries over the years, but two resonate most.

Video: Federer Subdues Nishikori in Shot-Making Spectacle

The iconic Roger-Rafa rivalry with Rafael Nadal and the electric Federer vs. Novak Djokovic rivalry.

"The rivalries, they're unique in many ways. Obviously the more I play Novak, the more he's achieving in that time frame, the more special it becomes," Federer told the media at the ATP World Tour Finals. "It's natural because every match becomes more important that we didn't play, as well.

"With Rafa I go back further. The fact is obviously the epic Wimbledon finals. Then we had all these matches for me which were hard, the French Open finals. But they were very important for my career as well to win the French Open in 2009."

In episode 43 of the Roger-Novak rivalry, Federer played some of his most dynamic tennis of the season defeating Djokovic at the World Tour Finals on Tuesday. The third-ranked Swiss snapped the top-ranked Serbian's 23-match winning streak and seized a 22-21 lead in their head-to-head series, that includes Djokovic victories in the Wimbledon and US Open finals this season.

Federer opened this month defeating archrival Nadal, 6-3, 5-7, 6-3, in the Basel final to capture his record-extending seventh Swiss Indoors championship. He snapped a five-match losing streak to Nadal, who still leads the iconic rivalry, 23-11.

So which rivalry is most special to the 17-time Grand Slam champion?

"Which is more special? It's hard to predict," Federer said. "Maybe slightly Rafa just because he came before Novak. I've always enjoyed the matchup with Novak. I think it's always very cool and athletic and straightforward, unlike Rafa where I feel like I have to change my game completely to compete with him."

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