Serena Williams the latest star sitting out Indian Wells

The BNP Paribas Open was already operating at a major marquee deficit, but Tuesday No. 1-ranked Serena Williams added to the tournament's miseries.

Williams withdrew without even stepping on the court, becoming the fourth multi-Grand Slam singles titlist to sit this one out.

With the abrupt retirement of Ana Ivanovic on the cusp of the new year, there are 10 active women's players who know what it feels like to hoist a Grand Slam singles trophy. Only seven of them have done it multiple times. Only three are in the field at Indian Wells: Venus Williams (7) No. 1 seed Angelique Kerber and No. 7 Svetlana Kuznetsova, who both have two.

Four of that elite class will be missing in action -- for very different reasons.

In a statement, Serena said a left knee injury -- which first surfaced at last year's US Open -- has prevented her from training properly.

Maria Sharapova is still serving a 15-month drug ban, Azarenka, the defending champion, is the mother of a 2-month-old baby boy, and Petra Kvitova is recovering from stab wounds she suffered during a home burglary.

"For Azarenka, it's a joyous occasion," Shriver said. "For Kvitova, your heart aches for her. Sharapova -- it's just so disappointing. That should have been within her control.

"Missing these multiple major winners, you look at the field at Indian Wells, and it demonstrates the strength of women's tennis. Hopefully, those champions all come back soon."

Here's a look at the other top stars who are missing in action and what to expect from each:

Maria Sharapova

The five-time Grand Slam champion will be the first one back in action, beginning on April 26, at the Porsche Tennis Grand Prix in Stuttgart, an event she has won three times. It's the first of three wild cards, preceding Madrid and Rome, she has been granted.

Sharapova will have just turned 30. Of note, Serena has won an astounding and unprecedented 10 Grand Slam singles titles since reaching that milestone.

A savvy marketer, Sharapova has been busy on social media, sending along visual progress reports from her workouts at the IMG Academy in Bradenton, Florida. She played a Las Vegas charity event in October and a December exhibition against Olympic gold medalist Monica Puig before 12,000 in San Juan, Puerto Rico. There were promotional events for her candy company, Sugarpova, in Moscow.

More recently, she made a splash at the Vanity Fair after-Oscars party, wearing a cobalt blue spaghetti strap gown from David Koma (with accessories by Jack Vartanian and Jimmy Choo).

She will start out with zero ranking points, which means in the early going she'll be dependent on wild cards or have to qualify her way into tournaments. Which raises an intriguing question: Will Roland Garros or Wimbledon, where Sharapova broke through as a 17-year-old, grant her that wild card if she isn't yet ranked among the top 100 or so?

Victoria Azarenka

After her son, Leo, was born on Dec. 20, 2016, Vika said she would be back "pretty soon."

There's been no update on that time frame for the return of the two-time Australian Open champion, but it could be later in the 2017 season.

Azarenka is one of the few players in the game who can sometimes compete with Serena. Although she's 0-for-11 in Grand Slams and the Olympics head to head, Azarenka has won four of their nine other finals. Last year's 6-4, 6-4 win in the Indian Wells final was a good example of her lack of fear.

Going forward, there's no reason Azarenka can't win more Slams. She's still only 27, and there is a precedent for major success as a mother. Kim Clijsters notably won three Slams (of the five she entered) after giving birth to a daughter.

Petra Kvitova

The grisly December attack on the two-time Wimbledon champion in the Czech Republic town of Prostejov made headlines around the world -- and echoed the tragedy of the Monica Seles incident in 1993.

Kvitova immediately underwent surgery on her dominant left hand and has been working hard to rehab the damage to her fingers. But she still can't forcefully grip a racket.

According to her publicist, Katie Spellman, Kvitova recently flew to Lanzarote, one of the Canary Islands off the coast of West Africa, to begin some light fitness work. This is encouraging news.

While the original prognosis was for a six-month recovery period, that was the best-case scenario. Spellman says that assessment hasn't changed -- but that is for a return to practice, not competition. So Wimbledon, where she won in 2011 and 2014, is most probably out.

If she is able to fully recover, Kvitova has all kinds of time to do some damage. She's only 26, and that huge lefty serve will keep her in matches, particularly on the grass at Wimbledon.