NEW YORK -- Until now, 28-year-old Maria Sharapova could be counted on to fill in what gaps perennially top-ranked Serena Williams might have left open.
But early this week, Sharapova shocked sports fans worldwide when she announced she had failed a drug test at the Australian Open, leading to her provisional suspension effective March 12. As it stands, Sharapova's future is uncertain.
As for Williams, she is facing her own challenges, having played just one tournament so far this year as a result of a knee and upper-respiratory ailments.
These developments raise the question: Is the WTA melting down and about to lose all its leaders and bankable Grand Slam stars?
"I'm not concerned," Williams said at a news conference promoting her exhibition match against Caroline Wozniacki at Madison Square Garden on Tuesday. "We have a lot of great stars on the WTA Tour, a lot of people that play great tennis, a lot of great personalities like Caroline. We need that. We need those stars. We have a lot of players, a lot of U.S. players, and I think when the time comes, someone will definitely step up and be that leader."
True, every year somebody is going to win Wimbledon, the US Open and every other event. Whether that player(s) will do it convincingly and consistently, and in a way that generates interest, is another matter. So far, the players who seemed on the cusp of breaking through to true stardom have crashed, a trend that, ironically, goes all the way back to Wozniacki.
She finished No. 1 in the world for two consecutive years (2010 and 2011) without winning a Grand Slam event, a shortcoming that led to a lot of criticism. These days the consistency that enabled Wozniacki to pull off her singular rankings feat looks impressive.
While Wozniacki might not be the one to fill this emerging hole on the WTA Tour, perhaps one these other players will. Let's take a look:
No. 2 Angelique Kerber
She will want to show that her win in Australia was no fluke.
However, the aftermath from that championship has been inauspicious for Kerber. In the following event, she lost to No. 73 Saisai Zheng in Doha.
"I think you have days like this sometimes," Kerber told the press after that loss. "I don't know how many mistakes I did today. This is not my game; that's all I can say."
No. 3 Agnieszka Radwanska
She lost the 2012 Wimbledon final to Serena and reached No. 2 thereafter. Since then, she's had fluctuating results. Her existence in the tennis landscape was best captured by the comment she made after she won the 2015 WTA Finals in Singapore, becoming the first women to lose two round-robin matches and claim the trophy.
"A few weeks ago, I didn't even know I'd have a chance to be here, and there you go," Radwanska said to the press at the time. "It's the greatest day of my life."
No. 4 Garbine Muguruza
Many pundits invest a lot of hope in Muguruza as the next best hope for the WTA. At 22, she's the youngest reasonable candidate. She beat Serena at the 2014 French Open and lost to the world No. 1 in the 2015 Wimbledon final.
Muguruza played well for the rest of 2015, but she's off to a so-so start this year. Her combination of size, mobility and easy power seems ideal for today's game. Still, Muguruza faces the same challenge as many of the other 20- 25-year-olds on the tour: That is figuring out just what to expect of themselves in a culture of lowered expectations.
No. 5 Simona Halep
Her aversion to the pressure of expectations seems to be as much a part of her game as those wonderful wheels on which she motors around the court on a good day. Halep promised a great deal when she played a great French Open final against Sharapova in 2014. But Halep has failed to get as far as the third round in half of the six majors since and has played some truly disappointing big matches.
No. 9 Petra Kvitova
She owns two Wimbledon titles (2011 and 2013). Just 26, she may have more upside than any of the other contenders. The problem for the six-foot lefthander is that she tends to simply fall off the radar for long periods and still has a tendency to go to pieces against a player of any level at any time.
No. 12 Venus Williams
A 35-year-old icon, she doesn't appear like she'll ever be a factor in a major tournament. Venus hasn't played in a Grand Slam singles final since 2009. However, she will generate a lot of headlines this week as she returns to Indian Wells for the first time in 15 years.
No. 15 Victoria Azarenka
She looked like a major threat to Serena's dominance when the Belarussian won back-to-back Australian Open championships in 2012 and 2013. Injuries, a complicated personal life and a knack for losing rather than gaining traction at the most of critical times have turned her into something of an enigma.
This was never more apparent than at the Australian Open earlier this season.
"Mentally I didn't commit myself enough today," Azarenka said after falling to Kerber in the quarterfinals. "I didn't push myself into making a difference. I was too flat."
Others, including former Grand Slam champs such as Samantha Stosur and Ana Ivanovic, have seen their hopes fade. Sabine Lisicki's big serve carried her to the Wimbledon final in 2013 but fell apart when she met fellow surprise finalist Marion Bartoli (who has since retired). Lisicki hasn't really been the same since.
It could be especially beneficial to the WTA at this moment if one of these contenders not only located her game but held on to it in the coming weeks and months.