The No. 1-ranked Nadal went straight to the practice courts and worked on his serve, later saying it needs work to improve his chances of winning the Australian title.

The 24-year-old Spaniard led, 6-0, 5-0, when Marcos Daniel retired because of a left knee injury early today.

Andy Murray, the 2010 finalist, advanced when Karol Beck retired because of 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, when Beck quit, was the only man to beat Nadal in a Grand Slam tournament last year.

Nadal sympathized with Daniel, recalling he had had to retire from the last Australian Open because of an injured knee in a quarterfinal loss to Murray. But Nadal recovered quickly and won the next three majors and is now aiming to be the first man since Rod Laver in 1969 to hold all four Grand Slam trophies at the same time.

“It’s a terrible feeling, for sure. I wish him all the best for a fast recovery,’’ Nadal said after the match. “Last year was a really difficult time when I played this tournament. I came back here playing well . . . but to go out like that was hard mentally.

“Finally, I had a very good season in 2010.’’

Good season is the understatement of the tournament. Nadal won the French Open, then Wimbledon, and finally claimed his first US Open title. With Tuesday’s victory, he’s now on a roll of 22 consecutive match wins in Grand Slam tournaments.

“I came back after a difficult situation,’’ he said. “Winning Roland Garros was very important, it allowed me to play the rest of the season with a little bit more calm.’’

Still, he’s not thinking about victory celebrations at Melbourne Park just yet.

“I never think about winning the four Grand Slams in a row because that’s very far right now,’’ he said.

Daniel had a medical timeout at the end of the first set and had his left knee heavily strapped. He said he had injured his knee earlier in the week in practice, but thought it would get him through the match.

He said playing Nadal on center court made it more difficult.

“That’s the hardest part. In the beginning I was feeling embarrassed a little bit,’’ he said. “I tried to play the game, it didn’t work. It looks like I’m 75 years old.’’

When the Brazilian had a break point in the second game of the second set, he raised both arms and received a rousing cheer from the crowd. When Nadal had a rare miss with a wild overhead that sailed over the baseline to give the Brazilian a second breakpoint chance, the crowd went wild again.

Nadal finally held after the game went to deuce five times and it progressively seemed only a matter of time before Daniel would retire as he limped around.

“Save energy or not, I think the way that the match came doesn’t make a big difference,’’ Nadal said. “Is difficult . . . to say I played really well, I played bad. I think I played [all] right.’’

Hoping a third time will be luckier, Vera Zvonareva began her bid to reach a third consecutive Grand Slam final with a commanding 6-2, 6-1 win over Sybille Bammer in the day’s opening match on Rod Laver Arena.

Zvonareva, who lost to Serena Williams in the Wimbledon final and to Kim Clijsters in the US Open final last year, dominated the first set against Bammer and tightened her grip on the match in the second. She 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.

Also advancing were French Open finalist Sam Stosur, No. 12 Agnieszka Radwanska, and No. 7 Jelena Jankovic.

On the men’s side, No. 15 Marin Cilic of Croatia beat American qualifier Donald Young, 6-3, 6-2, 6-1, to advance along with No. 10 Mikhail Youzhny.