When Maria Sharapova steps out in her Swarovski crystal-encrusted candy apple red dress, the statuesque 20-year-old Russian won’t be attending a movie premiere or late-summer gala. She’ll be serving aces and screaming through backhands on her quest to win a second U.S. Open title.

At the U.S. Open, which began yesterday at the USTA Billie Jean King National Tennis Center in Flushing Meadows, fashion is a major part of the game. The Open has a looser dress code than other annual Grand Slam competitions, such as Wimbledon. As a result, it has become a place for players to express themselves through their clothing choices, the global tennis sports marketing director for Reebok, Dianne Hayes, said.

“The days of women wearing traditional collared shirts are really over,” she said.

The infamous “little black dress” worn by Ms. Sharapova at last year’s Open, which she won, may have turned heads, but the latest creation is even racier. The ensemble’s angular, shift-silhouette and striking scoop back neckline is in sync with this season’s fashion trends, many of which will make their debut at New York’s Mercedes-Benz Fashion Week, which overlaps with the U.S. Open. Ms. Sharapova’s coordinating day dress, similar in shape, is a black-and-white getup that recalls the elegance of a young Audrey Hepburn. Yet it shares something with its evening counterpart: Both dresses depict the New York skyline with Swarovski crystals.

Ms. Sharapova can also add racquetbag designer to her repertoire. During her first match, she will unveil the first piece of her limited edition Sharapova line of bags made in collaboration with Prince. The black patent leather bags are splashed with fuchsia and have compartments for a cell phone, MP3 player, makeup, and keys, among other items.

Other tennis players are pushing the fashion envelope as well. Nicole Vaidisova has been doing double duty as a tennis player and clothing designer. The 18-year-old Czech who favors flirty ensembles has two collections that she helped design exclusively for the U.S. Open, one for day matches and one for night matches.

Ms. Vaidisova favors “very feminine silhouettes,” Ms. Hayes said, noting thatone of the frocks Ms. Vaidisova will be sporting during the day is an aquamarine racer-back dress with gold binding and a gold foil bow just below the bust.

Reebok is outfitting other players, including the no. 3 seeded Jelena Jankovic, who has her own collections for the competition, as well.

Venus Williams last week launched her first fashion line, called EleVen, with the retailer Steve & Barry’s. She’ll be sporting the collection’s V-Court sneakers during her matches at the U.S. Open.

And more and more of the younger players, such as 19-year-old Ana Ivanovic, are wearing pieces that look fresh off the runway. Ms. Ivanovic will be wearing pieces from Adidas’s Adilibria collection, which uses layering and long silhouettes to create a feminine look. The Serbian player will look more bound for a soirée than a tennis match in a dark grape skort accented in gold with a thick waistband and mesh pleats, and a matching shirt with mesh cap sleeves.

According to the merchandising director of Grand Central Racquet, Joan Dziena, companies such as Nike and Adidas will tell retailers what tennis players will be wearing in advance of the season so the stores can have the outfits in stock.

Tennis fashion, however, doesn’t only apply to the players. Polo Ralph Lauren has designed the official uniforms for the umpires and ball boys and girls of the U.S. Open. All are wearing polo shirts in French navy with yellow accents from the RLX Tennis line. The linesmen and lineswomen are wearing chino bottoms. The ball boys are wearing matching navy shorts, while the ball girls are gracing the courts in French navy skirts with yellow ribbon stripes.

And style isn’t the only consideration. Most brands incorporate sports technology into their fabrics. In the case of Adidas, that includes Clima-Cool, which draws heat and sweat away from the body, and ForMotion, a technology that allows for better movement through three-dimensionally engineered fabrics.

“The past few seasons it’s been fashion and function,” a spokeswoman for Fila, Lauren Mallon, said. “Two key trends are more feminine styling and a comeback of classic styling.”

And people are willing to spend big bucks on outfits their favorite players are wearing. “This past season I had a lot of the tennis clothing that Maria Sharapova and Serena Williams were wearing,” Ms. Dziena said. “I was taken aback because the dresses that Sharapova was wearing were $90, and these girls were willing to come and take them right off the rack.”