Who's Most Likely to Step Up and Be Serena Williams' Top Challenger in 2016?

Handout/Getty Images
Angelique Kerber walks ahead of Serena Williams as they prepare to play in the final of the 2016 Australian Open.

Serena Williams ^[1] left the door open for another WTA Tour player to make a run at the No. 1 ranking. However, few seem ready to take on the challenge.

When the Qatar Open ^[2] in Doha gets underway this weekend, No. 2 Angelique Kerber will be the top seed. This gives Kerber and others a chance to make up some ground on Williams' 3,545-point lead.

But will they take it? So far, Williams has been able to skip tournaments and hold onto the top spot without a serious threat to her throne.

If players are waiting around for Williams to step aside before they make a move, that's not how the great ones rise to power.

Martina Navratilova didn't make way for the Steffi Graf era. Instead, Graf pushed through.

We've been living in the "last days" of Williams' career for the past two to three years. Yet Williams remains the queen of tennis, and no one has emerged as a clear successor.

There's been a revolving door at the No. 2 spot. Kerber holds it today. She took it from Simona Halep, who grabbed it from Maria Sharapova ^[3] last year. Before Sharapova, Victoria Azarenka ^[4], Li Na ^[5], Petra Kvitova and Agnieszka Radwanska too k turns.

Meanwhile, Williams has spent 157 consecutive weeks at No. 1 ^[6], second only to Graf (186).

Who is most likely to challenge Serena Williams in 2016?

Despite a nagging knee injury, age and withdrawal from four of the last five tournaments she was entered in, Williams has lost little ground.

Williams hasn't played a match since she lost to Kerber in the finals of the Australian Open. She's played in just two official WTA Tournaments in five months. She withdrew from Dubai and Doha, as well as the exhibition Hopman Cup.

When the season started, Halep was ranked No. 2 and Garbine Muguruza, No. 3.

With Williams, Sharapova and Kerber out of Dubai, Halep and Muguruza stood a chance to make a move. Both lost their opening matches in Dubai. So did two-time Wimbledon champion Petra Kvitova. In fact, all eight seeds lost ^[7] their opening match in Dubai, a WTA record.

Because Kvitova, Halep and Muguruza had points to defend, after sitting out of Dubai, Williams increased her lead over all three.

Williams has won eight of the last 17 Grand Slams. Seven other women—Li Na, Maria Sharapova, Victoria Azarenka, Petra Kvitova, Flavia Pennetta, Marion Bartoli and Kerber—have won a Slam since the beginning of 2012. Three of them, Li, Pennetta and Bartoli, retired.

Graf's former coach ,, told CNN's Open Court ^[8] that if Williams wasn't in the picture, "10 to 15 girls have the ability to win grand slams."

Well, Williams is still around so chances are the road to a Slam title goes through her. Who poses the biggest threat to Williams this year?

The following are the leading contenders.

Angelique Kerber

Handout/Getty Images

Angelique Kerber hugs the trophy in the locker room after winning the 2016 Australian Open.

Kerber looks healthier and fitter than ever. She proved she can beat Williams when it counts. However, can she repeat the feat? Championship talent usually announces itself before age 28. But Kerber could go on a late-career run. She's added more offense to her counter-punching play. Her confidence is sky high.

After winning the Aussie Open, Kerber seemed as astonished about her No. 2 ranking as she was her win over Williams. She told the WTA Insider, "It sounds crazy to be the No. 2 player in the world. That's my highest ranking and I've reached it at the end."^[9]

Simona Halep

Halep surged to the top of the WTA by winning lesser tournaments. By 2014, she reached the French Open final and landed among the top five.

She's beaten Williams before. But she's producing uneven results. She's gone through multiple coaches and seems to suffer from a mixed bag of big-match anxiety and over-tinkering.

Maria Sharapova

Sharapova already had her run at supplanting Williams. She turns 29 in April. With nagging shoulder and arm problems, her days on the Tour may be as numbered as Williams.

Still, she's been one of the most dominant players on the tour. Despite her 18-match losing streak to Williams, Sharapova at least keeps getting herself in a position topple the queen.

Victoria Azarenka

Three years ago, Azarenka appeared ready to take over the tour. She had won two Australian Open titles ^[10] in a row and reached the final at the U.S. Open two years straight. But then the injury bug hit. A series of injuries ^[11] hampered Azarenka's comeback attempts, and she fell out of the Top 20.

When she started this season off with a win at Brisbane, without dropping a set, Azarenka looked like one of the players with the best chance of beating Williams. That was until she ran into a red-hot Kerber.

As much as people talk about Sharapova's lopsided record against Williams, Azarenka's (3-17 ^[12]) is nearly as bad.

Garbine Muguruza

Muguruza patterns her game after Williams'. Muguruza is aggressive and relentless. She upset Williams at the 2014 French Open and reached the finals at Wimbledon last year.

But lately, she's struggled with injuries and inconsistency. After losing in Dubai, Muguruza told WTA Insider Courtney Nguyen: "I'm not really finding my game, so I think I have to rest a little bit and concentrate again and work hard and eventually come back to the tournament to play again."^[13]

Belinda Bencic

Dmitri Lovetsky/Associated Press

Belinda Bencic hits a forehand during a match in St. Petersburg, Russia.

Bencic is 1-1 against Williams. So statistically, she poses a much bigger threat to Williams.

Bencic, 18, is the only teen ranked in the Top 75. She's already won two WTA titles ^[14] and notched a win over Williams last year in the Rogers Cup.

Mentored by Martina Hingis, Bencic has also worked with Hingis' mother. Bencic has supreme court sense and all-around game. However, a weak second serve leaves her vulnerable to being bashed off the court.

Charlie Eccleshare of the Telegraph cautioned that Bencic's "lack of power may see her bullied out of big matches as Hingis eventually was."^[15]

Agnieszka Radwanska

Radwanska will move up to No. 3 in the WTA rankings by Monday. A consistent and crafty player, she plays well enough to stay in the Top 10. But her game is no match for Williams'.

Like Bencic, Radwanska has solid court sense. She's a counter-puncher with a few tricks up her sleeve. But she lacks easy power. It's the difference between simply retrieving balls and dictating play.

Madison Keys

Ryan Pierse/Getty Images

Madison Keys hits a forehand during a match at the 2016 Australian Open.

Due to injuries, Keys has fallen back a bit. However, she has championship talent. She's got the big serve and powerful groundstrokes. What she needs is consistency and focus. In a feature titled "The Evolution of Madison Keys," Tennis ^[16].com's Steve Tignor wrote ^[17] about the young American's focus on consistency this season.

Tignor pointed out that "It's easy to understand her preoccupation with it. While she reached the semifinals at the Australian Open and the quarterfinals at Wimbledon last year, she reached the quarterfinal round at just two other events."

Petra Kvitova

Kvitova has to be one of the most inconsistent two-time Slam champions in tennis history. She's battled fatigue, mononucleosis ^[18], asthma and injuries.

She recently split with coach David Kotyza, who oversaw both her wins at Wimbledon. She's currently without a coach and on a self-discovery tour. She spoke with Sport360, about her evolution. "I just feel this step is important for me to find out what I want, what I need to do, and how I can play well again so I think it's a good word: evolution."^[19]

The Best of the Rest

Sloane Stephens and Eugenie Bouchard remain on a wait-and-see list of future champions. Stephens is ranked No. 22, and she picked up her second WTA title earlier this year. Bouchard is ranked No. 61. This month, she's spent more time on an NBA basketball court ^[20] than in a tennis tournament.

Stephens has beaten Williams before, but that's a distant memory. Bouchard is winless against Williams, and at No. 61, she could even run into the Grand Slam champion in the first rounds.

Bencic and Kerber seem to have the inside edge at challenging Williams.

If Bencic can improve her second serve, she's got the temperament, focus and talent to win Slams. She's already proved she can beat Serena. She nearly upset Sharapova at the Australian Open. She'll get better with time. What she needs is more power.

Kerber put together the right formula to beat Williams. She's the counter-puncher with enough offense to move Williams around and make her uncomfortable. If Williams brings her A game, she remains unbeatable. But in the last two Slams, Williams got caught with B and C material against players who brought their A games.

Unlike Roberta Vinci, Kerber is a Grand Slam champion. She's also No. 2 and with all the ranking points Williams has to defend, this gives Kerber a chance at the top spot.

With newfound confidence, tour experience and the No. 1 ranking within sight, Kerber is the player most likely to challenge Williams this year. Heck, she already has.