Naomi Osaka's rise spells trouble for Serena Williams and points to golden age for women's tennis

Defining the parameters of what makes a sporting era great is not easy. It is not only subjective, but some would argue undefinable - that is what makes elite-level sport so captivating.

There are though a few broad rules most would accept. Complete dominance by one team or individual is rarely popular, since inevitably such a situation breeds jealousy, resentment and boredom. The other extreme is where there is too much competition and seemingly no great individuals or teams. Yes, the unpredictability is exciting but there is normally a creeping suspicion that the flat structure is partly down to the absence of an outstanding candidate. Golf fans will empathise with this situation over the last five years.

Women's tennis meanwhile has veered between both of these extremes over the last 15 years - an era that can be broadly split into two sections. Either it's been the Serena Williams show, a story of crushing wins, minimal competition and grand-slam dominance. Or when Williams has been absent a series of understudies have grappled unconvincingly to take centre stage. To underline the ongoing absence of a credible challenger, during the period of January 2004 until the end of last year only one player besides Williams - Kim Clijsters in 2010-2011 - won consecutive majors.

Which brings us to Saturday, when Naomi Osaka, still only 21, altered the landscape by winning the Australian Open and becoming the first non-Serena player since Clijsters to claim back-to-back grand slams. In so doing she also became the world No. 1 and the first player since Jennifer Capriati in 2001 to back up her maiden grand-slam title win by capturing the next one available. It's early days but Williams would appear to have a genuine challenger at last - especially given the way Osaka made her look every one of her 36 years during last year's US Open final hammering.

Now 37 and with 23 major titles, it appears less of an inevitability that Williams will surpass or even equal Margaret Court's record of 24. Suddenly, the previously undisputed alpha female Williams can see predators circling around her in every direction.

Saturday's beaten finalist Petra Kvitova, for instance - a giant of the game regathering her strength after a period of enforced hibernation. A horrific knife attack threatened to finish Kvitova's career at the end of 2016, but as her run in Australia confirmed she has well and truly rediscovered the form that has taken her to two Wimbledon titles.

Besides Kvitova, there are other hunters poised menacingly. Angelique Kerber may have exited the Australian Open early but she can boast the pedigree of three major titles and a statement win over Williams at last year's Wimbledon. Simona Halep, Sloane Stephens and Garbine Muguruza are all grand-slam winners who will feel they should be capturing the biggest prizes over the next few years. Caroline Wozniacki will also be a factor, Victoria Azarenka can't be ruled out of a third major, and then there's Maria Sharapova - the WTA ice queen still capable of turning her opponents to stone.

In fact everywhere you look, there are intriguing sub plots and emerging talents who will be inspired by Osaka's success. Twenty-year-old Arnya Sabalenka is even younger than Osaka and blasts her shots so hard it's as if she's firing cannon-balls off her racket. The highly-rated and hugely popular Ashleigh Barty, 23, is almost the polar opposite in style - possessing a throwback game full of beguiling spins and slices. Both players are already only just outside the world's top 10. Teenagers like America's Amanda Anisimova and Bianca Andreescu of Canada meanwhile are already gaining traction, scoring high-profile scalps with their fearless approach.

Sport is unpredictable, but with so many talented players and arresting personalities it is hard to imagine a monopoly like Williams' over the coming years or the sort of interregnum that we have been left with when she has been absent.

With Osaka surveying her kingdom, Williams wanting her throne back and a stellar supporting cast desperate to make a land grab, women's tennis should be heading for a golden age.