By Jeb Blount

RIO DE JANEIRO (Reuters) - Doctors for the Belgian Olympic medical team suspect polluted water in Rio's Guanabara Bay is responsible for a serious illness that sapped the strength of Evi Van Acker, Laser Radial 2012 bronze medalist, her coach told Reuters.

Van Acker, who sits 10th overall after six of 10 preliminary races, was stricken with a severe gastro-intestinal illness during training in July and despite a course of antibiotics her strength and stamina remain compromised, said Wim Van Bladel, her coach in a phone interview in Rio.

Olympic organizers have faced harsh criticism for choosing Guanabara Bay as the sailing venue and for failing to meet promises to clean up sewage that contaminates the bay with bacteria and viruses and floating trash that threatens to slow or damage boats.

"The judgment of the medical team is that the water is the likely cause of her illness and continuing low-energy level. These diseases affect your muscles' ability to react," he said.

In a statement in Dutch, Belgian Olympic officials didn't mention the water, only saying Van Acker sickened several weeks ago, that her performance was compromised and she was undergoing an intense 36-hour treatment program.

World Sailing spokesman Darryl Seibel said that his organization reviews water quality tests from the Rio de Janeiro state government daily. A race or training session has never had to be canceled because water pathogen levels were above the World Health Organization standard for primary contact, he added.

The Rio de Janeiro Olympic Organizing did not respond to requests for comment.

Many sailors have said they have suffered no ill-effects and downplay any risks, suggesting that the water concerns are overshadowing some of the most exciting and challenging sailing of their lives.

However, Van Bladel is not convinced, saying that the impact of Acker's illness showed on Wednesday when Laser Radial sailors left the light-wind courses of Guanabara Bay for the large waves and heavy winds of the Copacabana ocean course outside the Bay.

"The courses yesterday were physically difficult, and her weakened condition showed," he said, although he added it is impossible to tell for sure if the water caused Van Acker's illness.

"I can't make that claim, he said. "But the organizers chose this place when they had cleaner venues. Now we just have to deal with it." .

Laser sailors interviewed after Wednesday's races, which lasted about an hour each, said they were "grueling."

After two second-place finishes in the first four races, Van Acker was fifth overall on Tuesday. She only finished 16th and 15th on Wednesday, dropping to 10th.

The top 10 in each class after 10 preliminary races qualify for a medal race. Sailors receive points equal to their finish with the lowest score wining. In the medal round points are doubled.

(Reporting by Jeb Blount, additional reporting by Julia Fioretti in Brussels and Benoit Tessier in Rio de Janeiro, editing by Neil Robinson)