Just now watching Serena's match and learned about her befriending a little 5 year old cancer patient named Jack. Serena didn't tell anyone, but was asked about it during a press conference.
This was the only article I could find.
Jack and Serena met on Twitter. She decided that if she made it to Wimbledon she would bring him and his family to see a match. They spent parts of Wednesday and Thursday together and Jack now calls Serena his "best bud".
How very sweet!!