U.S. Open Players to Watch By Geoff MacDonald and Vijai Singh 2013 U.S. Open: Players to Watch: Among the players to watch at this year's United States Open are John Isner, Jelena Jankovic, Juan Martin del Potro, Dominika Cibulkova, Grigor Dimitrov and Sloane Stephens. A laconic Argentine nicknamed the Tower of Tandil, del Potro has the most powerful ground game in men's tennis. At 6 feet 6 inches, del Potro serves cannons, but his sweeping flat forehand is his favored weapon, and his two-handed backhand is rock-solid. In 2009, del Potro stunned the tennis world when he overpowered Roger Federer to win the United States Open. A series of injuries stymied his career, but he has steadily climbed the rankings and is once again viewed as a contender for major titles. His epic encounter with Novak Djokovic in the Wimbledon semifinals was the match of the tournament. Although Djokovic won, the sustained high level of play announced that del Potro, seeded sixth at the Open, was back at the top of his game. This summer, del Potro won the Citi Open in Washington, and he reached the semifinals of the Western & Southern Open near Cincinnati. Del Potro's resurgence deepens the pool of possible winners at Flushing Meadows. John Isner The 6-foot-10 Isner defeated three top 10 players - Richard Gasquet, Novak Djokovic and Juan Martín del Potro - en route to the final of the Western & Southern Open, where he lost in two tiebreakers to Rafael Nadal. Isner's new coach, Mike Sell, has him playing with unbridled aggression, running around second-serve returns to wallop his big forehand. He is also using his high-bounding kick serve to pull opponents off court. His improved quickness and movement have been vital to his resurgence on the North American hardcourt circuit. A key for Isner, seeded 13th at the Open, will be recovering physically after his successful but grueling summer schedule. If he can survive the early rounds without being extended in lengthy matches, Isner is capable of beating any player in the draw. Grigor Dimitrov The prodigiously talented Dimitrov, a 22-year-old from Bulgaria, has steadily climbed through the rankings, and he will be the No. 25 seed at the Open. In 2008, Dimitrov won the junior titles at Wimbledon and the United States Open, but his results were uneven until this year. He began working with the Swedish coach Mikael Tillstrom, who has emphasized fitness and more disciplined shot selection. The hard work on the practice court has transformed Dimitrov from a talented shotmaker to a dangerous opponent capable of beating anyone. He upset Novak Djokovic in three sets on red clay in Madrid, a result that signaled his rising level of play. His game is modeled on Roger Federer's, with a flowing one-handed backhand, a rhythmic service motion and a dominating topspin forehand. Dimitrov, who extended Rafael Nadal to three sets at the Western & Southern Open, is a sublime talent and a joy to watch. Sloane Stephens Stephens, a 20-year-old American, showed serious competitive mettle in her three-set upset of Maria Sharapova in the Western & Southern Open, using superb court coverage and ability to absorb pace to turn the match her way. Watching Stephens handle the escalating pressure of higher expectations with an ebullient grace has been a delight for fans. In addition to having a powerful all-court game, Stephens has learned how to grind out a tough win when she is not playing her best, a quality she displayed at Wimbledon this year on her march to the quarterfinals. She also reached the Australian Open semifinals. The 16th-seeded Stephens has the speed, the power and the mentality to go deep into the second week at Flushing Meadows, where she is a crowd favorite, and she loves the big stage. Dominika Cibulkova A 24-year-old from Slovakia, Cibulkova is a fearless ball striker whose ability to generate astonishing pace belies her diminutive stature. At 5-fot-3, she is built like a gymnast, with a powerful lower body that helps her cover the court with one of the quickest first steps in tennis. Cibulkova, seeded 18th, uses her speed to go on the offensive at every opportunity, taking the ball early off both sides as she looks to take time away from her opponent and dictate play. She plays energetic, up-tempo tennis, and revels in ripping the cover off the ball with her blistering forehand. She trailed Agnieszka Radwanska, 3-6, 2-4, in the final of the Bank of the West Classic at Stanford, Calif., last month before putting on a stunning display of aggressive shotmaking to turn the match around and win in three sets. Jelena Jankovic She held the No. 1 ranking for a short period in 2008, and she was a mainstay in the top 10 for five years. Jankovic's ranking fell in the last two years because her consistent, counterpunching style did not have the power to punch back against the game's big hitters. Much like Caroline Wozniacki, another former No. 1 criticized for playing an outmoded defensive game, Jankovic has not had a big weapon to win a point. Now 28, Jankovic has had a splendid summer, reaching the Western & Southern Open semifinals with victories over Sabine Lisicki, Ekaterina Makarova, Sloane Stephens and Roberta Vinci, all ranked in the top 25. A speedy Serb, Jankovic has beefed up her forehand and begun attacking the net on short balls. She has also added a well-disguised drop shot. But she has not forsaken her signature strengths: she changes direction on her ground strokes at surprising times and with accuracy, and she competes for every ball with unflagging tenacity.