It’s hard to believe, but Serena Williams may be human after all.

Despite being the most recent grand slam winner, taking out her sixth US Open title by beating Caroline Wozniacki last September, Williams looked a weary woman in Perth during the Hopman Cup.

She struggled past Flavia Pennetta (needing to order a coffee mid-match) and Lucie Safarova, was beaten by Agnieszka Radwanska and annihilated by Canadian starlet Eugenie Bouchard.

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Williams turns 34 in September, and incredibly won her first major more than 15 years ago.

Along the way she’s seen off rivals like her older sister Venus, Justine Henin, Maria Sharapova, Jennifer Capriati and Kim Clijsters.

Soon, however, she will no longer be a fixture at the business end of grand slam tournaments.

Belinda Bencic. The second coming of Martina Hingis? Photo: Getty

World No.2 Sharapova will no doubt be happy to see the back of her, given she has won just a paltry two of their 18 career meetings.

But after Sharapova, who are the next crop, the women in line to inherit the mantle as the most dominant players in the game?

We take a look at four of the women most likely.

Belinda Bencic

Bencic is a 17-year-old Swiss prodigy, a former junior world No.1 who made huge strides in 2014, culminating in her first grand slam quarter-final appearance at the US Open. Along the way she defeated Angelique Kerber and Jelena Jankovic. Having started the year ranked 184 in the world, she enters the Australian Open at 32. It may take her a few years to put together her best consistently, but Bencic is an exciting prospect, and has the game to trouble the top women on her day.

Key quote: “I improved my results always. I played second round of Australia and third round of Wimbledon and now quarter-finals here (US Open), so it’s always gone up. I think it’s really positive that I take the small steps, but it goes up always.”

Can Garbine Muguruza win a major? Photo: Getty

Eugenie Bouchard

No surprises here – the 20-year-old Canadian lit up the tennis landscape in 2014, making the semis at Melbourne Park and Roland Garros, before losing to Petra Kvitova in the Wimbledon final. Bouchard is a gritty competitor, and a sports marketing guru’s dream. She’s a can’t-miss prospect, and it appears only a matter of time before she breaks through for a major and becomes world No.1.

Key quote: “I am very motivated to win a grand slam. It’s been a lifelong dream of mine. I feel like I’ve taken steps in the right direction to achieve that. This year I’ve been close in every slam, so I’m just going to keep going.”

Garbine Muguruza

The Venezuelan-born Spaniard won her first WTA title in Hobart last year, in an impressive return from ankle surgery that ruined the second half of her 2013. She made her first grand slam quarter-final at Roland Garros, where she destroyed Serena Williams and was on the cusp of beating eventual champion Sharapova. Muguruza, 21, enters the Australian Open in good form, having beaten Sara Errani and Agnieszka Radwanska in Sydney.

Karolina Pliskova showed her pluck in last year’s Australian Open. Photo: Getty

Key quote: “You know, it’s very difficult for me to tell you how mentally I can be stronger, but next time I’ll have more experience. If I can again reach the quarter-finals in a grand slam, well, I’ll try and be even more courageous to hit each point even more strongly.”

Karolina Pliskova

Pliskova has had little of the fanfare surrounding the other names on this list, but has been a steady improver and looks set for a breakout year. The tattooed Czech made five WTA finals in 2014, and defeated Ana Ivanovic at the US Open and Angelique Kerber in Nuremberg. Last year at the Australian Open she had a tough second-round loss to Daniela Hantuchova (12-10 in the third, in 41-degree heat). The 22-year-old upset former world No.1 Victoria Azarenka in Brisbane last week.

Key quote: “I definitely feel better. During this year I improved really a lot. I won so many close matches like this and so many matches against better players.  I got two titles, so I think I’m a better player than I was last year.”