Roger Federer firmly believes that there should be strict measures for anti-doping violations.

Following the announcement of Maria Sharapova’s two-year ban, the top seed in Stuttgart called for "zero tolerance."

"I didn't get into all the details,” he told reporters at the tournament. “But I am for zero tolerance ... She of course has the right to defend herself, like anyone ... Whether it's intentional or not, I don't see too much difference. You must be 100 percent about what you are taking, [and] know its effects and consequences. If that's not the case, you must be sanctioned."

Sharapova tested positive for meldonium, a substance that was not banned until the beginning of this year, at the Australian Open. She said she had taken it for many years for medical reasons and had not known it had been banned.

An independent anti-doping tribunal found that Sharapova had not known of the ban and did not intend to commit an anti-doping offense, but questioned whether her recent use of the substance had been for medical reasons. It also said that the five-time Grand Slam champion did not adequately communicate her use of the drug.

Federer has regularly backed increased anti-doping efforts in tennis. He also repeated his call for retroactive testing.

“We should keep blood samples for 10, 15 or 20 years for scaring potential cheats,” he said.