Grand Slam king Roger Federer says the toughest test Serena Williams faces in her quest for the calendar Grand Slam will come off court.

The 17-time Grand Slam champion believes withstanding the growing pressure and scrutiny that mounts with each match—rather than match-play itself—will be Williams' biggest obstacle to overcome. 

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"The hardest part is actually not the playing, but the constant talk about it," Federer told ESPN's Darren Cahill when asked his view of Serena's Grand Slam pursuit. "It's not like she's only one match away. She's still what four matches from here and another seven at the U.S. Open. So we're still a long way to go. She's got a very tough draw. She clearly is not going to think further than playing Venus next. Everybody talks about the calendar Grand Slam, I've even been asked, which shows how intrigued everybody is about it."

Despite the burden of history, Federer said he believes Williams can capture the calendar-year Grand Slam.

"You would think that maybe Wimbledon and the U.S. Open would be easier for her than the Australian Open and the French so that's why this whole dynamic [is at play]," Federer told Cahill. "But it must be tough for Serena and she knows how hard it will be for her to achieve it. But she can do it."

Asked to assess the state of rival Rafael Nadal's game in the aftermath of Nadal's second-round loss to qualifier Dustin Brown, Federer said he expected a very competitive match.

The eight-time Halle champion, who watched Brown upset Nadal in Halle last summer, credited the German-Jamaican with playing a "crazy match", but believed Nadal would prevail.

"Clearly, I'm aware that he's not around anymore," Federer said of Nadal. "Dustin Brown played a crazy match. The thing is I knew it was going to be hard going into it because I had watched the entire match in Halle last year in Germany when they played and Dustin beat Rafa. It was not that much of a surprise because Rafa had come right from the French Open so he had zero preparation going into Halle and Dustin played this crazy match again.So I figured he was going to have a chance this year, but over five sets probably not get the win.

"For Dustin, it was an unbelievable effort. For Rafa, clearly he's got some work to be done, especially in practice. That's usually how he finds his way back. I guess the French Open is the one he wanted more than here. Lately, he's been struggling here at Wimbledon. I don't exactly know why. The first week has been tough here."

Photo credit: Wimbledon