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 on Tuesday, 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.Rafael Nadal returns to Marcos Daniel during their first round match at the Australian Open in Melbourne on Tuesday. APOnly 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. If you do something and you let him win a game, it is worse.”advertisementNadal will play American qualifier Ryan Sweeting, who beat Daniel Gimeno 6-4, 6-4, 6-1.Another one of those top five that Daniel mentioned was Andy Murray, the 2010 finalist who also advanced when Karol Beck retired with a shoulder injury in the third set of their first-round match. The fifth-seeded Murray, who was leading 6-3, 6-1, 4-2, was the only man to beat Nadal in a Grand Slam tournament last year, the quarterfinals at Melbourne Park.”You’d rather finish the match off without your opponent being hurt,” Murray said, “but it does happen quite a lot. So you just have to move on and get yourself ready for the next round.”Fourth-seeded Robin Soderling had to go the distance but was rarely challenged, completing a 6-4, 6-2, 6-2 win over Potito Starace of Italy. Australian wild-card entry Bernard Tomic advanced to the second round with a 6-3, 6-2, 7-6 (5) win over Jeremy Chardy of France. Tomic could meet Nadal in the third round.Another winner was 2009 U.S. Open champion Juan Martin del Potro, who played only three matches last year due to a right wrist injury. He beat Dudi Sela of Israel, 7-6 (13), 6-4, 6-4.”I don’t want to think in the past,” del Potro said. “I won a Grand Slam, but I’m working to improve my game. I don’t know if I can play like two years ago or not, but I will try.”Jo-Wilfried Tsonga, who lost in the final here to Novak Djokovic in 2008, came back from two sets down to beat Philipp Petzschner of Germany 4-6, 2-6, 6-2, 6-3, 6-4. Andreas Seppi of Italy beat Arnaud Clement of France 3-6, 2-6, 7-5, 6-3, 6-2 in another late match.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.The U.S. Open champion never allowed the 2009 Australian Open finalist to get into the match, placing pinpoint forehands to all areas of the court. When Safina did have an opportunity to return, her many unforced errors gave away the point to Clijsters.”I expect my opponent to come out and play their best tennis,” said Clijsters. “She obviously didn’t do that today.”Safina was less polite.”I was sitting in the changeover, and I was like, OK, at least how can I get a chance to hurt her?,” Safina said. “There was nothing that I could do to hurt her. Embarrassing.”Elsewhere, No. 15 Marin Cilic of Croatia beat American qualifier Donald Young 6-3, 6-2, 6-1 to advance along with No. 7 David Ferrer, No. 10 Mikhail Youzhny, No. 11 Jurgen Melzer, No. 20 John Isner, No. 31 Feliciano Lopez and No. 32 Guillermo Garcia-Lopez.Former No. 1 Lleyton Hewitt and David Nalbandian of Argentina were scheduled to meet in a later first-round match in a replay of their 2002 Wimbledon final won by the Australian.Hewitt leads the series 3-2 but has not beaten Nalbandian since their controversial 2005 Australian Open quarterfinal in which the pair bumped into each other purposely on a changeover. The video clips of that altercation have featured prominently on television promotions of the night Rod Laver Arena match.advertisementAfter the 2005 match, won in five sets by the Australian, Nalbandian said Hewitt was “not a gentleman'” and “nobody is friends” with the Australian. Hewitt said Nalbandian was “not the cleanest guy.”No. 2-ranked Vera Zvonareva began her bid to reach a third consecutive Grand Slam women’s final with a 6-2, 6-1 win over Sybille Bammer. Zvonareva lost to Serena Williams in the Wimbledon final and to Kim Clijsters in the U.S. Open final last year.She dominated the first set against the 30-year-old Bammer, conceded only four points in the first four games of the second set and didn’t allow the Austrian to hold until the sixth game.Ana Ivanovic, the 2007 French Open champion and 2008 Australian Open finalist, slumped to her worst result in seven years at Melbourne when she lost 3-6, 6-4, 10-8 to Russia’s Ekaterina Makarova. The 19th-seeded Ivanovic saved five match points before finally going out.Also advancing were French Open finalist Sam Stosur, the fifth-seeded Australian who beat American wild-card entry Lauren Davis 6-1, 6-1; No. 12 Agnieszka Radwanska, who took six of the last seven games after a medical timeout in the third set to hold off Japanese veteran Kimiko Date Krumm 6-4, 4-6, 7-5; No. 7 Jelena Jankovic; No. 10 Shahar Peer; No. 13 Nadia Petrova; No. 18 Nadia Petrova, No. 22 Flavia Pennetta; No. 25 Petra Kvitova and China’s Peng Shuai.