"I'm extremely angry and disappointed. I had to lose my career and never opted to cheat no matter what," tweeted former world No. 1 Jennifer Capriati, in a long series of posts attacking Sharapova.

Aries Merritt, a U.S. hurdler, said there was no excuse for Sharapova to be unaware that WADA added meldonium to its latest list of banned drugs effective Jan. 1, which it circulated to competitors.

"As an athlete it is your responsibility to always know what's being placed on the banned list. Period," said Merritt at the U.S. Olympic Committee summit in Los Angeles.

Former U.S. Olympic swimmer Aaron Peirsol told Reuters there was some confusion over meldonium. "It wasn't even anything that was illegal until the beginning of this year," he said. "You have a bunch of people taking stuff that the athletes probably don't even know (is illegal)... it becomes gray."

Sharapova said she had not read an email informing her that meldonium was banned for use in sport.

She will be provisionally suspended from playing tennis from March 12 and could be prevented from competing for Russia at the Rio de Janeiro Olympics this year.

Some Sponsors On The Fence

Sharapova's deal with Tag Heuer, owned by French luxury goods group LVMH, expired at the end of 2015, and the company said on Tuesday it had dropped renewal talks in view of her announcement.

Nike Inc, the world's biggest sportswear brand, and German luxury car maker Porsche, a unit of Volkswagen AG , have said they are suspending their relationship with Sharapova as they gather more information and wait for a decision on a ban.

A person close to Sharapova told Reuters her team was encouraged that none of the sponsors so far have said they were terminating contracts with the player, although they had the right to do so.