Serena Williams says Maria Sharapova "showed a lot of courage" in taking responsibility for her failed drug test.

The 21-time major champion said Tuesday she "hoped for the best" for Sharapova, a day after the Russian star revealed she failed a test the day she lost to Williams at the Australian Open in January.

Sharapova said she tested positive for the little-known drug meldonium, which became a banned substance under the World Anti-Doping Agency code this year. The five-time Grand Slam champ could face a long ban from the International Tennis Federation, possibly ending her season and preventing her from competing at the Olympics.

Williams plays good friend Caroline Wozniacki on Tuesday night in an exhibition at Madison Square Garden.