Sloane Stephens has made the third round or better at all four Grand Slam tournaments. (Chris Hyde/Getty Images)
The WTA's next generation is starting to make an impact. Fifteen of the tour's top-100 players are 20 or younger, including one player who has made a Grand Slam semifinal, another who has defeated three major champions and a third who is being touted for her top-10 potential despite turning 18 only two months ago.
Here are my top-10 prospects in the 20-and-under crowd, based on results (which weighed heaviest), pure talent and potential. I should add one caveat: If I haven't seen them play, I didn't put them on the list.
1. Sloane Stephens
Country: United States
Current ranking: 16
Notable result: 2013 Australian Open semifinalist
Stephens has hit a rough patch since her breakthrough in Melbourne, but she's still the youngest player in the top 20. Even putting aside her Australian Open semifinal, Stephens has posted the most consistent results of anyone in her age group at Grand Slam tournaments, as she made the third round or better at all four before turning 20. Few players can match her raw athletic ability, and she is capable of both grinding out victories and outhitting opponents.
Whether it all comes together depends on if she can stay healthy and keep her head on straight when faced with adversity. Can she maintain her focus in those moments? Also, her physical gifts can make her lazy with her footwork, and she's not as explosive as she could be. It will be interesting to watch Stephens the rest of the year as she adjusts to being under the spotlight.
2. Laura Robson
Country: Great Britain
Current ranking: 38
Notable result: Beat three major champions -- Kim Clijsters (at the U.S. Open, Li Na (U.S. Open) and Petra Kvitova (Australian Open) -- as an 18-year-old
Before Stephens made her big run in Australia, Robson was the most accomplished teen. The 2008 junior Wimbledon champion sent Clijsters into retirement with a second-round upset at the 2012 U.S. Open. She followed that up by stunning Li Na, an arguably more impressive feat given the 2011 French Open champion's form at the time. Robson made her first WTA final last September, at the Guangzhou Open, and she helped open up the draw for Stephens in Melbourne this year when she knocked off Kvitova, the 2011 Wimbledon winner, in the second round.
Robson is the cleanest ball-striker of the young set. But, much like another left-hander, Kvitova, she can be horribly streaky. One bad shot can derail her entire game as the frustration quickly sets in and she completely unravels. She can definitely improve her serve and limit her double faults. Her lack of overall consistency is the biggest red flag, and the hope is that she'll relax more as she gains experience. Having more faith in her fitness might help her confidence deep in matches.
3. Madison Keys
Country: United States
Current ranking: 63
Notable result: Made two WTA Premier quarterfinals (Sydney International and the Family Circle Cup) and the third round of the Australian Open this year
If this list were based solely on upside, Keys, who turned 18 in February, would be at the top. Elite players such as Li and Agnieszka Radwanska have already taken notice of her game, which includes a world-class serve, and predicted that she could be a top-20 or even top-10 player soon. This is Keys' first full season on the WTA Tour after playing under an age-restricted schedule since she turned pro in 2009. Right now, it's just a matter of getting matches and earning experience. She'll play her first French Open in May.
4. Heather Watson/Christina McHale
Country: Great Britain/United States
Current ranking: 39/55
Notable result: Watson: Won the Japan Open last October for her first WTA title, ending a 24-year drought for British women; McHale: Made the third round at all four majors
Mono has derailed both players' careers, with McHale diagnosed last summer and Watson just this month. Before that, Watson and McHale were two of the most consistent in the younger group. Watson is one of the few young players to have won a WTA title. McHale reliably won early-round matches and was capable of a big upset, such as victories against Caroline Wozniacki and Kvitova in the first half of last season.
How their careers will develop is tough to say. Watson, who is generally undersized and underpowered, is best known for her resilience and fight in tough three-set matches. But she's yet to notch a a big, notable victory against a high-quality opponent. That said, she did beat Stephens twice last year. Can Watson's body withstand her grinding style? We'll see.
As for McHale, there's nothing flashy about her game, but it's already proved to be effective. She was knocking on the door of the top 20 last year and had transformed into a fierce competitor. But she hasn't rediscovered her game since her mono diagnosis (McHale is 6-9 in 2013).
5. Garbine Muguruza
Current ranking: 79
Notable result: Made the fourth round of the BNP Paribas Open as a qualifier and beat Caroline Wozniacki 6-2, 6-4 at the Sony Open to advance to the fourth round
Muguruza was an unknown last year when she took a wild card into Miami and proceeded to beat Vera Zvonareva and Flavia Pennetta to reach the fourth round. Now, Muguruza has quietly become a player whom the top seeds aren't crazy about seeing in their opening matches. With a game reminiscent of Anastasia Pavlyuchenkova's, Muguruza had Venus Williams on the ropes in February at the Brazil Tennis Cup before losing 7-5 in the third set, and she gave Sara Errani a three-set scare at the U.S. Open last year, too.
6. Kristina Mladenovic
Current ranking: 51
Notable result: Semifinalist at Open GDF Suez and Brazil Tennis Cup this year
Mladenovic -- who won the OEC Taipei WTA Ladies Open last November, the tour's first $125 event -- can pound the ball with her extreme forehand. Aside from that, her game doesn't exactly have balance. She is a solid doubles player with good instincts at the net, so it would be nice to see Mladenovic develop into a power hitter who can finish points with volleys. Otherwise, her movement and backhand can be exploited, and she racks up errors when her forehand is off.
7. Yulia Putintseva
Current ranking: 96
Notable result: Beat Christina McHale (at the Australian Open) and Laura Robson (at the Dubai Championships) in three sets this year
Putintseva was known for her over-the-top intensity as a junior, but the Russian-born grinder has mellowed out a bit as she's transitioned to the pro tour. She has garnered the attention of Martina Hingis and Patrick Mouratoglou as a potential future star, but, as they say, you can't teach height. The 5-foot-4 Putintseva doesn't have the power of, say, a Dominika Cibulkova, another diminutive player who makes up for it by punching well above her weight class, so we'll see how her game develops over time. She's frustrating to play against and her attitude can get under opponents' skin. It's actually really fun to watch.
8. Genie Bouchard
Current ranking: 92
Notable result: Made the Family Circle Cup quarterfinals this year after victories against Robson and Sam Stosur
The 2012 junior Wimbledon champion could easily do for Canadian women's tennis what Milos Raonic has already done for the men. Bouchard has developed later than her best friend, Robson, having opted to continue to play the junior Slams up to last year. I was impressed watching Bouchard at the Family Circle Cup. Bouchard is 5-foot-10 but favors placement and spin over big hitting, and though she doesn't have any big weapon of note, her game is simply solid all around. She's a calm competitor who doesn't make a lot of errors. I'd like to see her crank up the aggression as she matures, but she seems to be on the right track.
9. Monica Puig
Country: Puerto Rico
Current ranking: 102
Notable result: Pushed Angelique Kerber (at the Brisbane International) and Venus Williams (at the Family Circle Cup) deep into a third set this year
Talk to Puig for a few minutes and she'll tell you she's kind of a big deal in Puerto Rico, a country with little tennis tradition but a huge sporting culture. Though she's trained in Miami nearly all of her life, Puig still has that "big fish in a small pond" attitude, that thing the kids call "swagger." She showed her competitiveness in narrow losses to Keber and Venus.
Puig clearly believes in her talent and thinks she can stand toe-to-toe with anyone. I like the attitude. She possesses a solid all-around game with some good power, but everything needs to be tightened up to make her a consistent threat. Look for her during the U.S. summer hard-court season. She might be ready to pull off that big upset after a few close calls.
Monica Puig gave Angelique Kerber and Venus Williams tough matches this year. (Tertius Pickard/AP)
10. Taylor Townsend
Country: United States
Current ranking: 351
Notable result: Beat No. 57 Lucie Hradecka at Indian Wells in March for her first WTA main-draw victory
There's so much to like about her game. The former junior No. 1 is essentially the Radwanska of this generation, one of the few young players who seems to take pride in winning with variety and attacking the net. The problem right now is that she doesn't come close to having the movement necessary to play that style. The good news is that she's working on it. When I saw her in Indian Wells and Charleston this year, she was significantly fitter than last summer. She knows she has to keep improving in that area.
The most glaring absences from my list are two talented and buzzworthy Croatians, Donna Vekic, 16, and Ana Konjuh, 15. Konjuh turned heads when she won the Eddie Herr International and Orange Bowl late last year. She then won both singles and doubles at the Australian Open this year to become the No. 1 junior. The hype for Vekic, meanwhile, has been going for years, and she's already ranked No. 82. I simply haven't been able to watch either player enough to assess their games. But they're two players to watch for and I'm sure they'll make me revise my top-10 list sooner rather than later.
Also keep an eye on Annika Beck (Germany, 19), Lauren Davis (United States, 19) and Ashleigh Barty (Australia, 17). Beck is already ranked a career-high No. 60 thanks to a flurry of ITF titles in 2012. For Davis and Barty, their small stature is the biggest concern as they spend more time on the pro circuit.
How would you rank the 20-and-under set? Leave your rankings and assessments in the comments.