A 21-year-old illegal immigrant charged with killing and sexually assaulting a 92-year-old grandmother in Queens is no longer facing separate charges of smashing his father with a ceramic mug.

Prosecutors on Thursday dropped the Nov. 26 case against Reeaz Khan stemming from a scuffle with his dad, noting he “is facing more serious charges” — a murder indictment in the Jan. 6 attack that killed Maria Fuentes in Richmond Hill.

Assistant District Attorney Dylan Nesturrick told Queens Criminal Court Judge Jeffrey Gershuny that Khan’s father “does not wish to go forward” with the domestic assault case.

Instead, Khan, an immigrant from Guyana, will return to Queens Supreme Court on Feb. 4 on the murder case. The indictment also charges him with manslaughter, attempted rape and sexual abuse.

Prosecutors said Khan came up behind Fuentes — described as a “cat lady” — on Liberty Avenue and 127th Street and knocked her to the grown and sexually assaulted her.

Khan bizarrely explained that he had bent down to help the woman when his pants fell down.

He was not in court Thursday for the dismissal of the earlier domestic violence case.

His lawyer, Samantha Tucker, did not speak during the brief proceedings and declined to comment to reporters as she left court.