Anwar was arrested by police on Wednesday and spent almost 24 hours undergoing questioning in connection with the alleged assault before being freed on bail.
He has refused a police request to undergo a DNA test, saying he fears possible tampering of his sample during investigations into an accusation that he sodomised a male aide earlier this year.
Anwar, officially an adviser to the opposition Keadilan party (Justice) party, has not been formally charged in the case but has to report back to the police once the bail expires on August 18.
One of Anwar's lawyers was quoted in The Star newspaper as saying that his client will not agree to a police suggestion for a foreign forensic expert to draw his DNA sample.
R Sivarasa, who is also Keadilan vice-president, said the police failed to understand Anwar's concerns over possible fabrication of evidence to frame him.
However he said Anwar was open to the idea of an international expert conducting a DNA test.
This is the second time in a decade that Anwar's political career has been jeopardised by such allegations.