Physically, Roger Federer thinks he is a better player now than he was when he was the No. 1 player in the world and winning 17 Grand Slam championships.

According to tennis.com, age isn't the reason the 33-year-old Federer isn't dominating the way that he did from 2004-07.

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Lack of a killer instinct is the reason.

"I had an unbelievable winner mentality," Federer told tennis.com. Not that I don't have it today, but I haven't won as much as I did back then. I think that could make a difference. Otherwise I'm very pleased with how things are going this year."

The website noted that Federer won 42 titles, including 11 Grand Slams from 2004-07 when he first ruled the tennis world. He's won 26 titles with five slams since.

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"So much time has gone by and I've practiced so hard over the years, that I feel I have more power on my serve. I volley better now, I guess," said the Swiss star. "I've gotten to understand so many things over those years. But the thing back then is I was so unbelievably confident. I was coming through stretches where I wouldn't lose against Top 10 players. I wouldn't lose finals."

The emergence of Rafael Nadal had something to do with Federer's lost grip on his dominance. Later, Novak Djokovic and Andy Murray joined tennis' elite.

But Nadal's recent struggles with injuries and Federer's comeback from injury is helping Federer turn back the tables in his favor.

Last year, back issues affected Federer, and he dropped to as low as No. 8 in the world. But he's back to No. 3 with three titles and a spot in today's quarterfinals against 20th-seeded Gael Monfils.

Federer's ability to move freely about the court without pain has been key in his resurgence.

"I think especially now it's been really excellent the last three matches here at the U.S. Open," he told tennis.com. "The coordination is there, as well. I feel like I've gotten used to the hard courts by now. It's really working well. I'm very pleased.

"I think when you keep yourself in shape and train the right way, that's how you do it. Then actually it's not such a surprise for yourself. But I'm clearly happy about it because it's become a game of movement."

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