How much will grass help Federer?

WIMBLEDON, England -- With 26 previous meetings between the two, there is not a lot of new ground for Roger Federer and Novak Djokovic to tread. But there remains one big gap to be filled when they face each other in the Wimbledon semifinals: The two have never met on grass, and their first encounter on this most traditional and idiosyncratic of surfaces promises to reveal new dimensions to their rivalry.

Though Djokovic comes in as the defending champion, it is the location of this meeting that puts the most shine on Federer's chances.

What will prevail, Djokovic's 6-1 record in their matches since 2010 or Federer's 6-1 edge in Wimbledon titles? Since Federer's historic, streak-snapping victory at the French Open last year, it has been largely one-way traffic, starting with the U-turn in their memorable five-set battle at the U.S. Open. Djokovic came from two sets down and saved a match point with a return forehand winner, and he has not lost a set to Federer since. The Serb defeated a tired Federer easily on the clay courts of Rome in May, and out-steadied Federer in a straight-sets victory at the French Open.

But this is grass, Federer's most successful surface and, if only for lack of sample size, Djokovic's least. Part of the fascination of Friday's contest will be seeing how the matchup plays out on the turf. Federer knows he must take the offensive in rallies, and the surface gives him a better chance of being able to hit through Djokovic's nearly impenetrable siege on the baseline. But the attacking approach also means increased risk of making errors.

"Obviously, it's much harder to defend on grass time and time again than on any other surface, you know," Federer said. "But it's hard to set up sometimes. Going flat through the courts, you're playing little with margins. We're used to playing with much more topspin and giving ourselves margin over the net, whereas on grass I think it's worth it to go closer to the lines, use a lot of the down-the-line shots, which aren't easier to pull off on other surfaces."

Paul Annacone, Federer's co-coach, told ESPN.com that he feels the grass is still playing fairly slowly, but with "no secrets" between the two, the key will be their level of play.

Staying on the offensive will be even more important for Federer given his back problems earlier in the tournament. After defeating Mikhail Youzhny in the quarterfinals, Federer offered his most detailed report in a BBC interview, saying that he had been able to "serve to all corners" and "move very well" but remained a "little worried" during the match and did not chase balls when he did not think there was a chance of winning the point.

Even that little bit of lingering concern creates a big question mark for Federer going into Friday's match, because being able to run from corner to corner and dig out balls will be crucial against Djokovic, who may be better than anyone else at moving opponents around.

Djokovic, meanwhile, does not have to worry about producing anything extra special, but rather avoid the slow starts and up-and-down play he has shown recently.

He is well aware of the threat Federer poses on this surface when he is playing well.

"He has great variety in his game," Djokovic said. "He uses his serve very well, He opens up the court. He uses that slice really well to get the balls to bounce low. He's very aggressive at times. He can defend well.

"I think that grass courts are suiting his style the most, so it's going to be an interesting match."

But Djokovic also pointed to his own strengths, made evident in winning the title last year. His victory over Jo-Wilfried Tsonga in the semifinals was particularly striking, showing Djokovic's ability to suffocate even some of the most athletic and inspired net-rushing seen in the game these days.

"I improved playing on grass in last couple of years," Djokovic said. "This year, knowing I won the title last year, in the back of my mind obviously gives me a lot of self-belief.

"I do try to be more aggressive a little bit, in the important moments go for my serves and shots. This is something that is really important on this surface."

Even since the matchup was set, trying to work out who has the edge has been a popular game on site and beyond. Even Federer agreed to take part, weighing Djokovic's recent Grand Slam dominance against his own history at this tournament.

"I think he's the slight favorite," Federer told BBC. "At the end, it doesn't matter. It's just going to play out the way it does."