Sharma, who had represented Mukesh in the case for a brief period, arrived in the court with signed undertakings by two of the accused appointing him as their lawyer on Tuesday.
Mukesh is the brother of Ram Singh, a prime accused in the case who allegedly hanged himself inside the high-security Tihar jail on March 11.
The advocate said that Mukesh's mother and brother had approached him on Tuesday evening and following a discussion, he had gone to meet Mukesh in Tihar in the evening. He said that while he was speaking to Mukesh, Thakur also expressed an interest to replace his lawyer with him.
Till now, V K Anand was representing the case of Mukesh while A P Singh that of Thakur. Both Anand and Singh were completely taken by surprise by the latest development. While the former refused to comment, the latter said he was only defending Vinay Sharma.
The trial of the Delhi gangrape case is underway at a fast-track court in Saket.
The four accused are on trial for raping and assaulting a 23-year-old student on a moving bus in New Delhi on the night of December 16. They also beat up the male friend of the victim, who died in a Singapore hospital on December 29. The fifth accused, a juvenile, is undergoing trial before a juvenile justice board.