A new era: Will 'big four' become two?

(CNN) -- They are the "big four" of men's tennis: Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray.

Between them they have won every grand slam title since the start of 2010, and 30 of the last 31 dating back to 2005.

However, there is a groundswell of opinion that four will become two in 2013, as No. 1 Djokovic and third-ranked Murray continue their ascendance while their older rivals fall away.

"With Nadal being seriously injured and Federer not getting any younger, I see the top two players being Djokovic and Murray, they have time on their side," six-time grand slam champion Becker told CNN ahead of the Australian Open.

Nadal has not played competitively since last year's Wimbledon in July, struck down by recurring knee problems. A stomach virus then prevented the Spaniard's comeback in 2013's opening grand slam in Melbourne, which starts on Monday.

It is the second extended injury layoff of the 26-year-old's remarkable career. After similar problems in 2009 he returned with a vengeance, claiming five more grand slams for 11 in total, including the French Open title last year for a record seventh time

Federer is still going strong in his 32nd year, and during 2012 regained his No.1 spot in the rankings after winning his record 17th grand slam by claiming the Wimbledon title.

Djokovic beat the Swiss maestro in the final of the ATP World Tour Finals in London in November as he went back to the top of the global tree, also beating Murray in their round robin group match.

Federer then upset home favorite Murray in the semifinals, and former world No. 4 Greg Rusedski told CNN that his performances during last season had "defied logic" after appearing to be on the decline in 2011.

Murray, buoyed by his gold medal victory over Federer in the Olympic final and his later U.S. Open success, was expected to beat him again at the O2 Arena, but instead was swept aside in their last four clash.

The Scot had been set to again battle with his longtime friend Djokovic, with whom he had a series of classic matches in 2012.

The first came in the semifinals of the Australian Open, beaten in five sets by the Serbian, who then overcame Nadal in another marathon six-hour encounter -- the longest grand slam final in history.

It was similar fare at Flushing Meadows in September, where Murray turned the tables for his first triumph in the tennis majors, but having to go the full distance after Djokovic battled back from two sets down in the longest final in the tournament's history.

And there was yet more to come at the Shanghai Masters, where the pair contested another final where the standard of tennis was of the very highest order.

Djokovic survived five match points to conjure up another remarkable win, but for Becker it was the signal of a possible changing of the guard.

"It was an absolute classic," the German legend said.

"It used to be Federer versus Nadal, now it's Murray versus Djokovic and they are both only 25 years of age."

Murray told CNN that the reason for such close matches is their familiarity with each other's game.

"We can anticipate what each other is going to do," he said.

The pair have been friends and arch-rivals since entering the junior circuits of the world at 11 years of age and Murray has seen his rival develop to become the best in the world.

Read: Can sports rivals really be friends?

"He has very few weaknesses, great ground strokes, serves well, his returns perhaps the best in the world, moves so well and he now has a lot of belief in himself," Murray said.

Even though it was at his expense, Djokovic was pleased for Murray when he finally snapped his grand slam duck at the U.S. Open because "many people had questioned his qualities."

"It was great to see that and hopefully our rivalry can develop over the next year," Djokovic said of Murray, who shares the same birth month -- May.

But others believe it would be dangerous to write off Federer and Nadal just yet.

Neil Harman, the tennis correspondent of The Times of London, said he was looking forward to normal service being resumed in 2013.

"It should never be underestimated what Federer has done for the game, and he shows no sign of going away," Harman told CNN. "And I'm looking forward to Nadal returning at his best ."

The rivalry of the big four and their near total dominance of the slams has helped fuel the incredible global interest in tennis despite the economic downturn, with increased prize money, commercial revenues and crowds.

Despite Nadal's absence, record attendances are predicted at the Melbourne Tennis Center over the next fortnight as Djokovic and Murray bid to live up to Becker's prediction and Federer does his level best to prove him wrong.

Via: A new era: Will 'big four' become two?