Tennis great Rod Laver wants to make something perfectly clear: If Rafael Nadal wins this month's Australian Open for a fourth major title in a row, it would be "a great effort" — but not a Grand Slam.

That's because a true Grand Slam is winning the Australian Open, French Open, Wimbledon and the U.S. Open in a calendar year, not just consecutively.

Laver would know. He accomplished the feat twice, in 1962 and 1969; no man has done it since.

The buzz at the Australian Open, which begins Monday, will be about Nadal, and his bid to become the first man in 42 years to win four straight Grand Slam tournaments. The Spaniard's championships at the French Open, Wimbledon and U.S. Open came in 2010, so if he wins in Australia, his "Rafa Slam" would stretch over two seasons.

"He's got three under his belt and he's playing well. There's a good chance he could pull it off. But it's not a Grand Slam, certainly," Laver told The Associated Press in a telephone interview from his home in California. "People will say, 'He's going for a Grand Slam.' And I say, 'No, he's not doing that.' That wasn't the way this whole thing was set up. It starts in January and ends in September; starts with the Australian Open and ends with the U.S. Open."

"Still," Laver continued, "what he's trying to do is a great effort. It's not a Grand Slam, but it's a great effort."

Laver, a 72-year-old from Australia, is surprised no one has swept tennis' top four tournaments since his day. Roger Federer came close — winning three in a row over two seasons on two separate occasions, and winning three in a year (Australian, Wimbledon, U.S.) in 2004, 2006 and 2007.

Jimmy Connors won three of the four in 1974; Mats Wilander did the same in 1988.

"It's uncanny that no one has put it all together," Laver said. "I thought that possibly someone like Boris Becker would have been able to do it. Or maybe Bjorn Borg. Or Pete Sampras. There were quite a few players that dominated their era but never were able to put it together and make a total Grand Slam of it."

And what about Federer, owner of a career-record 16 major titles?

"Federer was the latest one that I thought would definitely have a shot at it. He kept winning three in a year, except the French Open," Laver said, "because Nadal owns the French."

Asked why he thinks it hasn't happened since the 1960s, Laver pointed to the importance of being injury- and illness-free and the current depth of men's tennis.

Part of what makes his achievement special is that Nadal now has benefited from the downtime during the offseason. He also doesn't have to deal with the mounting attention and focus that would come with pursuing a real Grand Slam.

"The pressure of winning a Grand Slam — there's supposed to be a start and an end to it," Laver said. "There's no real start or end to it if you just keep going from one year to the next. You can say, 'Well, I'll start at Wimbledon,' or 'I'll start at the U.S. Open and win all four in a row.'

Make no mistake: Laver is a big fan of Nadal's and his style of play.

"The effort he puts in all the time. He never gives up on shots. He keeps improving his game," Laver said.

"You just marvel at how he can put so much spin on the ball and not make errors," Laver added. "You would think he'd be making errors somewhere along the line. He turns that racket over and puts so much spin on the ball — but he's so accurate, with so much depth. It's great to watch."