Ask tennis aficionados to name the greatest era in the men's game and they will come back with a range of suggestions. Was it the Twenties, when France's "Four Musketeers", René Lacoste, Henri Cochet, Jean Borotra and Jacques Brugnon, took on the Americans Bill Tilden and Bill Johnston? Maybe the Sixties, with its conveyor belt of great Australians, headed by Rod Laver, the last man to achieve the Grand Slam?
What about the late Seventies and early Eighties, when John McEnroe and Jimmy Connors fought Bjorn Borg and Ivan Lendl for world supremacy? Or the early Nineties, when a new wave of Americans led by Andre Agassi challenged Boris Becker and Stefan Edberg?
A common theme runs through every suggestion: tennis thrives on competition and needs great players to pit their talents against one another to produce the finest moments.
Today another classic era beckons. Roger Federer's duels with Rafael Nadal were already bearing comparison with the rivalry of Borg and McEnroe and now the game's two leading figures, acclaimed as arguably the greatest players ever on grass and clay respectively, find themselves in a gang of four.
Novak Djokovic announced his arrival among the elite with victory in the Australian Open 12 months ago, while the bookmakers' favourite to win this year's event beginning in Melbourne on Monday is Andy Murray, the world's outstanding player over the last six months. In that time the British No 1 has reached the US Open final, where he lost to Federer, and won four titles, including two at Masters Series level.
Paul Annacone, who coached Pete Sampras and Tim Henman, and now works for the Lawn Tennis Association, is thrilled by what lies ahead. "I think it's great for the game, great for the fans – and actually great for Roger as well," he said. "This is probably the first time in the last few years when he's realised that he just has to get better.
"Roger had pulled away from everyone else, with the exception of Rafa on clay. They were in a league of their own, with the next best players in a big pack behind them. Now everything's changed. With Rafa winning on grass, Novak winning the Australian Open and Andy having such a great run, it's the four of them who have pulled clear of the rest.
"Roger has to improve," Annacone added, "and I think he will. He's very competitive and very resilient. He needs to be a bit more effective. Andy has shown in his last few matches against him that if Roger's level drops he'll get beaten when he comes up against these players. A few years ago his level might have dropped now and then, but he would still win. I don't believe he can do that now."
Neither Federer nor Nadal can take anything for granted. In the last 18 months Djokovic has beaten the Spaniard four times and the Swiss twice, while Murray has beaten Federer in their last three meetings and ended his losing run against Nadal at the US Open.
The contrasting styles add a rich flavour to the mix: Nadal is the supreme strongman and most dogged of retrievers; Federer the ultimate stylist and a player who loves to attack; Djokovic a formidable baseliner whose level rarely dips; and Murray a gifted touch player and masterful tactician.
Djokovic and Murray are just 21 and Nadal a year older. At 27 Federer is the old man of the group, but he has a remarkable fitness record. Indeed, it is the other three whose physical condition might become an issue. Murray has had recurring problems with a bipartite patella (his right kneecap is in two parts), Djokovic has sometimes had trouble lasting the pace and Nadal's all-out style puts pressure on his suspect knees and large frame.
That is one reason why Sampras believes Federer will win the two Grand Slam titles he needs to break the American's all-time record of 14. "It takes so much work for Rafa to keep going, to stay healthy and to stay on top," Sampras said.
"One thing Roger has over him is that I think it takes him a lot less energy to stay on top than it does Rafa. It takes a lot of work for Rafa to win his matches. I think he'll be smart with his schedule in the next couple of years, to peak at the majors. He puts so much effort into each point that eventually something is going to break. He's an animal, but as strong as he is I think the body will take its toll. There's a certain grind that Rafa goes through, unlike Roger, who's a lot more fluid. His matches are a lot easier. It seems like there are no easy matches for Rafa. He works for every point. It's physically and mentally demanding for him because he seems to be strung so tight."
What does Sampras make of their two main challengers? "I think Djokovic is also there, though I feel he's a little more fragile, a little more up and down," the seven-times Wimbledon champion said. "Murray's right on the edge of breaking through and winning a major. I think he's got the game to do that. The US Open was a big breakthrough for him. Roger's experience came through in the final, but I'm impressed by Andy. He's competitive, he's a great mover.
"I don't see any reason why he can't win a major. He's one of the best players in the world. It's just self-confidence, believing he has the game to win a major. The game itself is right there. He belongs there, but it takes time. I could sense watching the US Open final last year that he didn't really believe he could win that match. If he was in that situation again I think he would have a stronger belief."
Nevertheless, given that Djokovic and Murray have won only one Grand Slam tournament between them, might it be too soon to be talking about a golden era? The veteran American broadcaster and writer Bud Collins, who will be working in Melbourne next week 52 years after covering his first Grand Slam tournament, said: "I think it is too early to state that, though the great thing is that all four players are very young.
"I just hope they will be in situations where they play each other a lot more. I think I would have to say the era of Connors, Borg and McEnroe, with Lendl right behind them, was the greatest and what I liked so much about it was that they seemed to be playing each other all the time."
Elsewhere around the world opinion is divided. "I think we could well be heading into a golden age," Chris Clarey, of the International Herald Tribune, said. "All the ingredients are there. I was talking the other day to Larry Stefanki [Andy Roddick's new coach] and he compared it with both the early 1980s and the early 1990s. I think the comparison with the 1990s is a good one. Becker and Edberg were dominant, but then they were challenged by that great wave of American players."
Philippe Bouin, the highly respected tennis correspondent of L'Equipe , believes it is easy to forget the strength of past eras. "In 1992, for example, you had a top 10 in which every single player had won or was going to win a Grand Slam – Courier, Edberg, Sampras, Ivanisevic, Becker, Chang, Korda, Lendl, Agassi and Krajicek," he said.
"We have four very good players at the moment, but only three of them have won Grand Slams. Murray looks like a great talent, but he hasn't won a Slam yet. We're certainly going into a very good age, not only because of the four currently at the top. There are others who could join them – Del Potro, Tsonga, Gulbis, Cilic, Gasquet. What we could have in a year or two is a really exceptional top 10, not just a top four."
The Italian journalist Gianni Clerici, who played at Wimbledon and has been writing about tennis for more than 50 years, believes it is impossible to compare eras but is relishing the present one. "Tennis tends to divide into periods where you had one or maybe two players dominant and then times when there were several players winning the biggest tournaments," he said. "I always think it's better for the game when you have more players in with a chance of winning. It's a great thing that you have four such good players at the top right now."
String quartet: How the world's top four measure up
World ranking: 1
Grand Slam debut: 2003
Grand Slam best: Won French Open 2005, 2006, 2007, 2008, Wimbledon 2008
Titles won: 31
Career prize-money won: $20,814,797 (£14.25m)
Strengths: Power of groundstrokes, return of serve, court coverage, resilience
Room for improvement: Serve, volleys
World ranking: 2
Grand Slam debut: 1999
Grand Slam best: Won Australia Open 2004, 2006, 2007, Wimbledon 2003, 2004, 2005, 2006, 2007, US Open 2004, 2005, 2006, 2007, 2008
Titles won: 57
Career prize-money won: $44,644,857 (£30.56m)
Strengths: Forehand, variation and consistency of serve, volleys, speed around court
Room for improvement: Backhand
World ranking: 3
Grand Slam debut: 2005
Grand Slam best: Won Australian Open 2008, runner-up US Open 2007
Titles won: 11
Career prize-money won: $10,511,877 (£7.19m)
Strengths: All-round game, consistency, athleticism
Room for improvement: Variation of tactics, net play, adapting to new racket
World ranking: 4
Grand Slam debut: 2005
Grand Slam best: Runner-up US Open 2008, quarter-finalist Wimbledon 2008
Titles won: 9
Career prize-money won: $5,684,904 (£3.89m)
Strengths: Backhand, return of serve, speed around court, variations of pace
Room for improvement: Forehand, volleys, attacking net