Andre Agassi made a bold prediction that World No.1 Rafael Nadal will win the coveted Grand Slam this season.

Agassi bustled through the Wimbledon grounds in that familiar pigeon-toed gait and predicted tennis fans could witness history in the making this year.

Agassi not only insists reigning champion Rafael Nadal can defend his title under the new retractable roof on Centre Court, he is convinced the world number one can win all four grand slams in the same year, a feat last achieved by Rod Laver in 1969.

Nadal has already won the Australian Open this year and begins the defence of the French Open he has won for four years in a row at Roland Garros a week on Sunday.

Agassi, who is the most recent player to win tennis grand slams on all surfaces, albeit never inside a calendar year, said: "A lot of things have to fall right. You have to be good enough to win all four and you have to get lucky.

"But I think this is the first time in a real long time we are going to have the opportunity for somebody to pull that off.

"If Nadal routines Paris as he has for a number of years and comes here with confidence and puts together two good weeks we could be looking at him in New York with a realistic opportunity of pulling off the unthinkable.

"I didn't think I'd ever see it, just because it seems impossible. But you have to respect the way he has gone about his game, his career, his physicality.

"When he won at Paris in his first year what he said spoke volumes about who he was. He said it was awesome to win the French Open but his dream was to win Wimbledon.

"People laughed and I was one of them. I never thought for a second that his game could ever translate but he executed.

"To see what Nadal managed last year was special. Watching that epic final (against Roger Federer at Wimbledon) probably transcends any other match ever played."

Agassi was sitting beside Britain's Tim Henman and Belgium's Kim Clijsters and looking forward to a practice session for tomorrow's Centre Court Celebration at Wimbledon.

Agassi's wife, Steffi Graf, will complete the quartet who will play a mixed doubles, a men's singles and a ladies' singles in a mini-tournament with the roof closed to test the air-conditioning system which supplies 143,000 litres of fresh air to the arena every second.

All 15,000 tickets for the event, priced at £35, were sold out within five minutes.

The event, which is being screened live on BBC2, will also see singing stars Katherine Jenkins, Faryl Smith from Britain's Got Talent, and classic boy band Blake perform during breaks in the action.

The irony of the roof being closed will not be lost on Henman, whose epic semi-final against Goran Ivanisevic in 2001 was interrupted by rain just when it looked as if was going to reach his first grand slam final.

Henman, sceptical when the roof was first mooted, said: "It may have helped. But the roof is a fantastic addition.

"My concern was the technology, the air management system. I was cautious that it might be difficult to play because there might be moisture and it would affect the grass, if not ruin it.

"But with the amount of technology and research that has gone into it everyone is quietly confident it will have the right outcome. For the players, fans and media it guarantees the play and that is a huge asset."

One argument against the roof is that it favours those scheduled to play on Centre Court, a band of players which is bound to include Britain's Andy Murray.

And while Agassi believes Nadal is favourite, he does not rule out a first grand slam for Murray at Wimbledon at the age of 22.

Agassi said: "I don't think it is too early for him to win. I have seen him grow mentally over the last number of years.

"His US Open final against Federer was a wake-up call. He realises he expects to win now. His best surface is New York and Australia but I would be surprised if he doesn't find a way to win here."

Agassi, who won unexpectedly at Wimbledon in 1992 against Goran Ivanisevic, also revealed that he has switched from Pete Sampras to Roger Federer when asked to select greatest player he faced.

"It was always Pete throughout the bulk of my career," said Agassi.

"Then when I played Federer in the finals of the US Open (in 2005) I was convinced at that point that the game hadn't seen somebody like that yet.

"Excluding the freak from Majorca (Nadal), Federer would possibly have won all the grand slams for a few years in a row. Pretty extraordinary."