Serena Williams says Maria Sharapova "showed a lot of courage" in taking responsibility for her failed drug test. On Monday, the Russian star revealed she tested positive the day she lost to Williams in the Australian Open quarterfinals in January. At a news conference Tuesday to promote an exhibition at Madison Square Garden, Serena didn't want to talk about the fact she had faced an opponent with a banned substance in her system. The 21-time Grand Slam champion said that like the rest of the sports world, she was "surprised and shocked" to hear the news about Sharapova Monday. "I think most people were happy she was upfront and very honest and showed a lot of courage to admit to what she had done and what she had neglected to look at in terms of the list at the end of the year," Serena said. Sharapova announced she tested positive for meldonium, a blood flow-promoting drug she said she had been taking for 10 years for various health issues. Meldonium was banned because it aids oxygen uptake and endurance, and players were notified of the changes in the WADA banned substances list in December. Sharapova claimed she simply neglected to click on the link. She could face a long ban from the International Tennis Federation, possibly ending her season and preventing her from competing at the Olympics. "It's just taking responsibility, which she admitted that she was willing to do and ready to do," Serena said. "Just hope for the best for everybody in that situation." Sharapova and Serena have had some barbed words for each other in the past off the court, but Tuesday, Serena was fully supportive. While Sharapova is one of her biggest rivals based on the Russian's results and five major titles, Serena has dominated the head-to-head series, winning the last 18 meetings over more than a decade. "As Maria said, she's ready to take full responsibility," Serena said. "She showed a lot of courage and a lot of heart. I think she's always shown courage and heart in everything she's done, and this is no different." Two-time Grand Slam finalist Caroline Wozniacki, who faces Serena in the exhibition Tuesday night, agreed with her good friend's assessment of Sharapova's announcement. Wozniacki acknowledged, though, the lengths that elite athletes typically go through to ensure they don't accidentally ingest banned substances. "Any time we take any medication, we double, triple and quadruple check, because sometimes even things like cough drops or nasal sprays can be on the list," Wozniacki said. "I think as athletes we always make sure to really make sure there's nothing in it." — AP