A court was told the elderly pensioner was walking through the famous Altstadt district when was confronted by the young man screaming “money, money!”
He is also said to have shouted “Keep silent, or death,” according to German media reports.
When he discovered she had no cash, the young man dragged her into a alleyway between two buildings where he raped her while choking her.
The defendant – known only as Souhayl M – then rifled through her handbag, stole her house key and wrote down her address.
The 19-year-old now faces up to 15 years in prison when he is sentenced later this month.
The elderly victim had to be hospitalised after the attack.
Her attacker, a Spaniard of Moroccan origin, was charged two weeks later after his DNA matched with that taken from the scene of another robbery he’d been arrested for last May.
Dusseldorf police were also able to construct a likeness of the suspect after a police officer recognised the man at a train station.
In his appearance before Dusseldorf District Court on Thursday, Souhayl admitted to carrying out the vile attack.
“When I got home, I could not believe what had happened,” said the defendant in his statement.
Souhayl, of no fixed address, said he was on cocaine, ketamine, and alcohol prior to the attack, and had set out to commit robbery not rape.
After glancing through his criminal record, however, the judge pointed to a case of suspected sexual assault back in Spain, which Souhayl denied.
Born in Morocco, Souhayl was reported to have had a history of being involved in petty crime.
He dropped out of school at 15 and joined a spin-off of the infamous US-based street gang, the Latin Kings.
In Spain, he was arrested several times, including for violent brawls, and eventually fled the country in 2016 after he was threatened with jail time.
In Germany, he quickly became known to police for a string of shoplifting incidents and other small thefts.
Souhayl is also charged with stealing 30 euros from a woman at a nightclub.
Having admitted to charges of rape, theft and bodily harm, the young man faces a prison term of up to 15 years when the trial resumes for sentencing.
His trial will continue on March 27.