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A European player belting tennis balls on Court 7 drew onlookers to his midday practice Sunday. Sprinting to his left, he hit a topspin forehand like he had done for as long as anyone could remember. He scampered to his right and smacked a two-handed backhand that jumped off the court. He is naturally right-handed but makes his living hitting the ball primarily with his non-dominant hand.

No, this was not a Mallorcan named Rafael Nadal. It was Jurgen Melzer, a 32-year-old Austrian and the 29th-seeded man in the United States Open. Melzer, a former top 10 player who won the ATP Tour event in Winston-Salem, N.C., last week, said he started playing tennis at age 8 and, like a lot of small children, began with two hands.

“I had to hold the left hand on the bottom,” he says. “Thankfully or luckily, I kept it in this hand.”

Over the past decade, Nadal has gotten nearly all of the attention pertaining to this rare breed of player, the ones who make their living hitting most of their shots while holding the racquet in their so-called weaker hand. Nadal’s uncle and coach, Toni Nadal, famously had the foresight to train his right-handed nephew to be a left-handed player, which is generally considered an advantage in tennis.

But there are other players who fall into this category and get far less attention for it. Being able to play this way on such a high level goes beyond hard work, heart or a savvy uncle. It starts in the brain.

Dr. Mininder S. Kocher, the associate director of sports medicine at Boston Children’s Hospital, said handedness is controlled by the motor cortex, in the rear portion of the frontal lobe.

“We see handedness typically revealed at 2 to 3 years of age and developed by 5 to 6 years of age,” said Dr. Mininder, who is also a professor of orthopedic surgery at Harvard Medical School.

But he added, “Handedness may not be fully integrated until 8 to 9 years of age.”

This helps explain Angelique Kerber of Germany, seeded eighth at the Open. Naturally right-handed, Kerber started playing at age 3 and obviously was not making a strategic decision about tennis at the time.

“I don’t know why; I just picked up the racket with the left hand,” she said. “I had more feeling with the left hand than with the right.”

Her parents noticed but did not step in.

“They just said O.K., keep it in the left.,” Kerber said. “They didn’t try to change it.”

Kocher is a natural left-hander who was converted to being a right-hander early on — “the nuns,” he explained. He said that up to 40 percent of the population is not strongly left or right handed. But people can be trained to override their natural handedness. But he does not necessarily recommend it.

“It is important to remember that the brain is still developing and is plastic until the early 20s,” he said.

Still, he added, “I think forcing someone who has a natural handedness to write or play tennis with their other hand may be counterproductive, as it may lead to learning issues, confusion and decreased performance.”

Even a partial list of tennis players who fall in to the “mixed handedness” category — a term Kocher said is usually more accurate than ambidexterous — is nothing short of impressive. And there is a twist: some of them are lefties who play right-handed. These include Marion Bartoli, the recently retired (perhaps) Wimbledon champion; Carlos Moya, a Mallorcan like Nadal and the 1998 French Open champion; Margaret Court, the career leader in Grand Slams singles titles; and Kimiko Date-Krumm, the 42-year-old from Japan who lost her first-round match at the Open on Monday.

The mixed-handedness can also be seen in some of the biggest stars in other sports, too: Larry Bird, LeBron James, C.C. Sabathia, Michael Vick. Phil Mickelson is even known as Lefty, even though he is right-handed. He learned to play golf by mirroring his right-handed father.