But she did not respond to subsequent messages. On Friday, after her quarterfinal loss at the Nürnberger Versicherungscup in Germany, Lepchenko refused to comment further.

Asked about her two-month absence from the tour, Lepchenko cited a knee injury. “It’s been going on since last year,” she said.

After Lepchenko’s 5-7, 6-4, 6-3 loss to Ekaterina Makarova of Russia on Tuesday in the first round of the French Open, the reported positive test was the subject of most of the questions at Lepchenko’s news conference.

“At the moment I have no comment on any of this,” she said. “I’m here just to answer tennis questions. If you have any questions about my match, I would gladly answer them. But otherwise, I just have no comments.”

The I.T.F. would not say whether there was a case involving Lepchenko.

“The rules of the tennis antidoping program do not allow us to confirm or deny that a player has tested positive until a final decision is made,” an I.T.F. spokeswoman said. “The only exception to this is if the player him- or herself acknowledges the positive test.”

Under tennis’s antidoping program, players are often given provisional suspensions while their cases are being adjudicated, so an absence from the tour without confirmation or denial of a doping case can stir speculation, with a recent precedent still very much in mind. In 2013, Marin Cilic cited a knee injury in withdrawing from Wimbledon and subsequently missing time on the tour, but it was later announced that he had tested positive for a banned substance and was serving a provisional suspension.

Roselyne Bachelot, a former French minister of sport, said in March during a French television program that Rafael Nadal had been serving a silent ban from the sport during a seven-month stretch in 2012 and 2013 when he was absent from the tour. Nadal, who had cited knee injuries for his absence, sued Bachelot for defamation in a Paris court in April, and the I.T.F. publicly sided with Nadal.