Serena Williams and changing times on the WTA Tour

— -- Eight years ago, Justine Henin abruptly retired. A seven-time Grand Slam champ, she had had enough of the grind and wanted to begin the next phase of her life.

Not only did the game lose its top-ranked player, it lost its semblance of order atop the rankings for years to come. Players such as Ana Ivanovic, Francesca Schiavone, Samantha Stosur and Marion Bartoli all reached Grand Slam glory then almost immediately fell back into the pack of occasional contenders.

It wasn't until Serena Williams began her latest reign of dominance in 2013 that order was restored again. The current world No. 1 won six of 11 Grand Slam events and ruled the tour almost every week.

And then last year's US Open happened.  Williams was stunned by Roberta Vinci in the semifinals of the tournament, where the American came within two wins of an calendar-year Grand Slam.

We didn't realize it at the time, but a heartbroken Williams' missed shot at rare history signaled the onset of more changing times.

Williams, who has captivated and enthralled tennis fans for the better part of her 21-year career, has come earthbound quickly since last September, winning only one title, in Rome.

Still, despite her struggles, the question as to who will challenge and ultimately become the next face of the WTA Tour are troubling.

To add to the tour's state of transition, five-time major champion Maria Sharapova is facing a two-year doping ban, and barring a favorable appeals verdict, won't return until January 2018.

Bottom line is the WTA's landscape is unsettling, just as it was after Henin retired all those year ago.

"If you think of it from a marketing standpoint, the tour definitely could use its new stars stepping up consistently and creating some strong rivalries, and it's time," ESPN tennis analyst Pam Shriver said.

Victoria Azarenka, who turns 27 in July and has won two Grand Slam titles, has reached four major finals. Before injuries caused prolonged absences, she was generally acknowledged as the one player both capable and mentally strong enough to topple Williams and Sharapova on a consistent basis.

And then there are the others such as Eugenie Bouchard and Sloane Stephens, who have been christened as the next great ones, but have yet to fully materialize.

"Three, four years ago, if you would have asked me, I'd have thought [Petra] Kvitova would win three or four majors," ESPN analyst Brad Gilbert said. "I thought [Belinda] Bencic would take a big step up this year, but that hasn't happened. And I thought three different times that Madison Keys would be a top-3 player."

The top up-and-coming name this moment is Garbine Muguruza, who showed no signs of fear in defeating Williams in the French Open final a few weeks ago. Muguruza, 22, also took out Williams in the second round of the 2014 French Open, and then played solidly before losing in last year's Wimbledon final to Kvitova.

"Muguruza's talent is there," Gilbert said. "If you tell me five years from now she's won three or four Slams, I wouldn't be surprised. But if you look at her record, she has been very inconsistent. Maybe this last win, with the way she played, showed a glimpse that she can be a multiple Slam winner."

While Williams is arguably the greatest player of all time, her dominance has also likely created an intimidation factor that has kept other players from consistently challenging her.

"Just by the nature of the sport, because it's mano a mano, you play against individuals, and it breeds players like Serena and now [Novak] Djokovic to dominate," ESPN analyst and tennis Hall of Famer Cliff Drysdale said. "It has always been there.

"It goes back to [Rod] Laver. Later, Pete Sampras had his era when he dominated, and Chrissie [Evert] and Martina [Navratilova] seemed like the only two players on the women's tour for a generation. But it's sad there has been only one dominant woman's player."

According to Shriver, it shouldn't take Serena's retirement to spur on the next group of consistent winners.

"[Monica] Seles didn't wait for [Steffi] Graf, and Graf didn't wait for Navratilova," Shriver said. "But this group of challengers has not been comfortable living with success, and I can't put my finger on why exactly."

Even if they do wait, Williams' retirement will not automatically create the next great player. Gilbert said he wouldn't be surprised if it took eight to 10 years before another potential 10-time Slam champion emerges.

"You have to have right stuff, the right game to be able to do it and the right mindset," Gilbert said. "It doesn't come that easily. Maybe there will be some 16-, 17-year-old where we go, 'Whoa, we didn't see that coming.' It's always a possibility. But right now, I don't see anyone that dominant."

While we're waiting, the WTA Tour has to be doing the same and hoping that next phenom doesn't exist in a vacuum.

"The men's tour is like buying products in glass, where you can see what you're getting," Drysdale said. "With the women, there's a certain joy in seeing who's going to win if Serena is not in the field. But somebody has got to restore some kind of predictability, someone to be more consistent, to restore some order -- and nobody has."