NEW YORK (REUTERS) - Serena Williams, the top-ranked player in women's tennis, said on Tuesday (March 8) she was shocked to hear her friend and fellow tennis star Maria Sharapova had failed a drug test, but she commended the Russian for showing courage in taking responsibility.

"As Maria said, she's ready to take full responsibility and I think that showed a lot of courage and a lot of heart," Williams told reporters at a briefing ahead of a match she will play on Tuesday night in New York.

Sharapova said on Monday that she had failed a drug test at the Australian Open due to a substance she was taking for health issues.

The 28-year-old Sharapova, a five-time Grand Slam champion and the highest-paid woman in sports, will be provisionally suspended starting March 12, the International Tennis Federation (ITF) said.