MELBOURNE, Australia — To get an assessment of how much better Rafael Nadal and Roger Federer are than most of the competition in men’s tennis, look no further than Marcos Daniel.

At the Australian Open today, Daniel was on the other side of the net from Nadal, who is attempting to win his fourth major title in a row — the “Rafa Slam.” Daniel saw, ever so briefly, why the top-seeded Nadal is among the best ever in the game.

Only briefly, because the Brazilian’s left knee gave out, forcing him to retire from the first-round match while trailing 6-0, 5-0.

“If you see all the top five guys, they improve so much,” said Daniel, who has also lost to Federer. “They are one step forward, faster than the others, they hit the ball harder than the others, they can stay very focused for four hours, different than the others. Imagine Federer and Nadal, they are the same.”

Nadal’s limited court time should hold him in good stead for the rest of the tournament, but he wasn’t counting his blessings as the result of another player’s misfortune.

“I am a professional and I try my best on every point,” Nadal said, explaining why he didn’t hold back in the match. “That’s the best thing, to respect the opponent in that situation. .”

Nadal will play American qualifier Ryan Sweeting, who beat Daniel Gimeno 6-4, 6-4, 6-1.

On the women’s side, U.S. Open champion Kim Clijsters did nothing to dent her growing status as favorite by routing fellow former No. 1 Dinara Safina 6-0, 6-0.