Billy Jean King believes Garbine Muguruza's French Open victory heralds a new era for women's tennis

The Spaniard produced a stunning display of power tennis at Roland Garros to defeat defending champion Serena Williams 7-5 6-4 before collecting the trophy from King.

At 22, Muguruza joins two-time Wimbledon champion Petra Kvitova as the only grand slam winners born in the 1990s.

Muguruza announced her talent to the world two years ago by handing Williams her heaviest ever slam loss in the second round of the French Open and then lost to the American in her first final at Wimbledon last summer.

King, a 12-time grand slam singles champion, said: “The changing of the guard is starting.

“She’s the one everyone’s been talking about, for two years anyway. The kid’s got such power and she wants it. Her backhand’s just phenomenal, and her forehand too.

“The best players in the world hit down the line off a cross-court (shot) better than the others, and what does she do exceptionally well? She hits down the line off a cross court. It’s harder to change direction, it takes more skill, so you always look for that. You look for the movement. She’s very good at moving in with small steps.

“Twenty-two is a perfect age. I won my first Wimbledon at 22. She’ll have a chance now to keep that up instead of winning big when you’re 17 and you can’t handle anything.

“You can just tell this is very good timing for what she’s going to handle off the court as well because now there’s pressures off the court that she’s never had.”

Williams’ victory over Muguruza at Wimbledon took her to within one of equalling Steffi Graf’s Open-era record of 22 slam singles titles.

And that is where she still stands after failing to get across the line in three successive tournaments.

At the US Open, when she was also going for the calendar Grand Slam, she lost in the semi-finals to Roberta Vinci, while Angelique Kerber showed Williams was now vulnerable in finals by beating her in Australia.

Having lost just four of her first 25 slam finals, losing two in a row shows, if nothing else, that Williams’ rivals are no longer daunted by facing her on the biggest stage.

Her position at the top of the rankings is not under any immediate pressure, with Muguruza now her closest rival, but Williams turns 35 in September and cannot stay there forever.

A thigh problem may well have contributed to her defeat, although she moved a lot better than in her previous two victories over Yulia Putintseva and Kiki Bertens and certainly did not play badly.

King is concerned about Williams, saying: “Something’s wrong with Serena. Her footwork was not what it should be.

“I don’t think it’s the 22, I just think she needs to get in a different place. I don’t know her that well any more but she didn’t look happy. I want her to be happy as a person. Forget the tennis. She doesn’t have the same vim and vigour.

“I don’t know if it’s physical but something’s not quite right. Maybe grass will pep her up a little.”