Distraught, the woman is believed to have fled the flat in tears, later sending a text message stating that what he had did had been wrong.
However, she decided against reporting the matter to the police, but changed course in October 2014 when he contacted her again.
The message, which was shown to the court, read: "I am so sorry for emotionally abusing you and manipulating you to do things you didn't want to do. I am really sorry for raping you. I am really sorry for hurting you. It's all my fault".
Prosecuting, David Povall told jurors that Ip, whom he described as "bright and capable", had forced himself upon the woman after she attempted to avoid his earlier advances on a nearby sofa.
"They were sitting on the sofa together and as they talked he obviously wasn't listening to her because he kept trying to grope her and remove her clothes," he added.
"She stopped him because she wanted to talk and moved over to sit on the bed to put some distance between them.
"That was unwise because he came over and forced her down onto the bed and started trying to remove her lower clothing.
"She started crying during the sex and he obviously noticed that because he asked her if she was all right - but continued in any event.
"She felt frozen as it was happening and she explained she didn't resist to stop him. The sex ended and she remained there for a short time, then they both left."
The court also heard that the woman had told her boyfriend about the incident four months later, but only felt confident to act when Ip allegedly sent the message acknowledging his guilt.
Povall said that a "very long delay" had ensued after the woman had reported the crime, meaning that the defendant was not charged until late in 2015. "That explains why we are dealing with a 2013 case in 2017," he added.
Ip, of Burcot Gardens, Maidenhead, Berkshire, denies rape.
The trial continues.