"Digvijay's letter to me does not merit a response. Congress seems to be completely rattled and baffled. That is apparent from Digvijay's letter.
"Digvijay seems Congress' last resort," Kejriwal, who heads India Against Corruption (IAC), tweeted.
Responding to Singh's remarks, Kejriwal's close associate Kumar Vishwas told PTI "we have only one thing to say to him. Get well soon."
In a letter to Kejriwal, the AICC General Secretary has described the activist as a "self-serving ambitious megalomaniac" having "streak" of Hitler in him.
Singh alleged that Kejriwal wanted him to recommend his name for being made a member of the National Advisory Council.