It was/is directed at Sharapova. I'm guessing she just found out she has a twitter. And I'm also guessing Maria saw this tweet based on what she just tweeted herself. Looks like Maria fans will be forever tormented by Serena.
But I don't understand why Serena would bite back like that. Maria's tweet was clearly in support of Vika/Serena.
Sabine Lisicki - Victoria Azarenka - Nastya Pavyuchenkova
Yanina Wickmayer - Laura Robson - Aga Radwanska - KiKi Mladenovic - Angelique Kerber
Simona Halep - Heather Watson - Julia Goerges - Kimiko Date Krumm - Lucie Safarova - Nadia Petrova