COUNTRY Spain CURRENT RANKING 1 WHY YOU SHOULD KNOW HIM With a victory at this month's Australian Open, the 24-year-old Majorcan can become the first men's player to hold all four Grand Slam titles at the same time since Rod Laver in 1969.
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COUNTRY Denmark CURRENT RANKING 1 WHY YOU SHOULD KNOW HER The 20-year-old managed to make No. 1 thanks to a tour-high six titles in 2010, but Wozniacki's legacy won't be secure until she bags that elusive first Grand Slam title.
3 of 20Andrew Burton/Getty Images
COUNTRY United States CURRENT RANKING 180 WHY YOU SHOULD KNOW HIM The 18-year-old fought through last year's U.S. Open qualifying draw before upsetting No. 15 Ivan Ljubicic in the first round. The youngest player in the ATP Top 250, he'll play in this month's Australian Open after winning the USTA's wild-card playoff in December.
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COUNTRY United States CURRENT RANKING 106 WHY YOU SHOULD KNOW HER One of several candidates to become the face of American women's tennis when the Williams sisters retire, the 18-year-old from Teaneck, N.J., has already won a main-draw match at the U.S. Open and played a night match on Ashe.
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COUNTRY Serbia CURRENT RANKING 3 WHY YOU SHOULD KNOW HIM The 23-year-old reached the quarterfinals or better at each of the four majors in 2010, but it was the Djoker's brave run to the U.S. Open final that redefined and reinvigorated his career.
6 of 20Stan Honda/AFP/Getty Images
COUNTRY Russia CURRENT RANKING 63 WHY YOU SHOULD KNOW HER Since ceding the No. 1 ranking to Serena Williams in 2009, the three-time Grand Slam finalist and younger sister of former U.S. and Australian Open champion Marat Safin has had a bumpy descent.
7 of 20William West/Getty Images
Juan Martin del Potro
COUNTRY Argentina CURRENT RANKING 258 WHY YOU SHOULD KNOW HIM Many experts proclaimed Del Potro the sport's next big thing after he outlasted Roger Federer in the 2009 U.S. Open final. But the 22-year-old has since dropped 254 spots in the rankings after a wrist injury sidelined him 10 months.
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COUNTRY Serbia CURRENT RANKING 17 WHY YOU SHOULD KNOW HER Ivanovic spent 12 weeks at No. 1 in 2008, then went nearly two years before winning another title. The 23-year-old Serb hopes the second-half spurt that vaulted her back into the top 20 will continue into 2011.
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COUNTRY Sweden CURRENT RANKING 5 WHY YOU SHOULD KNOW HIM The 26-year-old was runner-up at the past two French Opens -- notching a pair of historic victories over Rafael Nadal in 2009 and Roger Federer in '10 -- but he's still hunting for that first Grand Slam title.
10 of 20Paul Crock/AFP/Getty Images
COUNTRY Belgium CURRENT RANKING 12 WHY YOU SHOULD KNOW HER Henin came back from a 16-month retirement in January 2010 and immediately crashed the Australian Open final, where she bowed to Serena Williams in three sets. She's been inactive since hurting her elbow at Wimbledon, the lone Grand Slam title missing from her trophy case.
11 of 20Peter Parks/AFP/Getty Images
COUNTRY United States CURRENT RANKING 18 WHY YOU SHOULD KNOW HIM The Greensboro, N.C., native became internationally famous after playing in the longest match in history: an 11-hour, 5-minute victory over Nicolas Mahut at Wimbledon.
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COUNTRY Russia CURRENT RANKING 2 WHY YOU SHOULD KNOW HER For years prone to self-destructive emotional outbursts at critical junctures, Zvonareva made made dramatic strides in her mental game in 2010. She reached the final at both Wimbledon and the U.S. Open while surging up to second in the rankings.
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COUNTRY Czech Republic CURRENT RANKING 6 WHY YOU SHOULD KNOW HIM The 25-year-old followed up a surprise run to the French Open semis with a stunning upset of Roger Federer in the quarters at Wimbledon, where he fell in the final to Rafael Nadal.
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COUNTRY Russia CURRENT RANKING 18 WHY YOU SHOULD KNOW HER The three-time Grand Slam champion showed signs of improvement after a nine-month layoff for shoulder surgery, but didn't advance past the fourth round of any major in 2010.
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COUNTRY Germany CURRENT RANKING 67 WHY YOU SHOULD KNOW HIM The 2010 ATP Newcomer of the Year began the year ranked 254th and, two Challenger titles later, finished it inside the top 70.
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COUNTRY United States CURRENT RANKING 62 WHY YOU SHOULD KNOW HER The Marietta, Ga., native failed to capitalize on her charmed run to the 2009 U.S. Open quarterfinals, finishing her '10 campaign with a tepid 25-24 record in singles.
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COUNTRY Bulgaria CURRENT RANKING 106 WHY YOU SHOULD KNOW HIM A former top-ranked junior who won boys' titles at Wimbledon and the U.S. Open, the 19-year-old has already been touted as "the new Roger Federer" -- hype that only gained steam during the second half of 2010 when he catapulted from 360th to 106th in the rankings.
18 of 20Ryan Pierse/Getty Images
COUNTRY Belarus CURRENT RANKING 10 WHY YOU SHOULD KNOW HER An aggressive baseliner with a fearsome backhand, the 21-year-old Belarusian is poised for a breakthrough after climbing to No. 6 in the rankings in 2010.
19 of 20Nick Laham/Getty Images
COUNTRY United States CURRENT RANKING 629 WHY YOU SHOULD KNOW HIM Still an amateur, the Lincoln, Neb., native won the U.S. Open junior title in September. Keep an eye on Sock as he earns his stripes on the Challenger and Futures tours.
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COUNTRY United States CURRENT RANKING 113 WHY YOU SHOULD KNOW HER The big-serving 19-year-old, who won the 2008 U.S. Open girls' championship without dropping a set, is the niece of former NBA All-Star Kiki Vandeweghe.
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