The law professor, on trial for corruptly obtaining gifts and sex from a 23-year-old student in exchange for improving her grades, said in court today that a senior anti-graft investigator had threatened him.

Tey Tsun Hang, who is defending himself, said Mr Teng Khee Fatt from the Corrupt Practices Investigation Bureau (CPIB) threatened him in Hokkien during a May 17 conversation.

Tey claimed that Mr Teng, the bureau's deputy director of investigations, said: "I stab you once, you die beautifully, with your legs straight but if you insist, I will stab you tens of times (and) you will die a most horrendous death".

Mr Teng, however, rejected that suggestion and responded by saying: "That is not true, sir.

"I only realised that Prof Tey is able to speak in Hokkien on May 18, and his Hokkien accent comes from - I don't know which Chinese province or area - that sometimes I do not understand him."

In proceedings today, Tey also tried to show that three statements he made to Mr Teng had all been "oppressively taken", to which the latter disagreed.

This is the ninth and last day of the trial.

Tey, 41, faces six charges of corruptly obtaining gifts and sex from former student Darinne Ko Wen Hui, in return for showing her favour in school.

The gifts include a $740 Montblanc pen, a $160 iPod touch and two tailored shirts worth $236.20. She also paid for a $1,278.60 dinner Tey hosted.

The court has heard that Ms Ko lost her virginity to Tey in his National University of Singapore (NUS) office on July 24, 2010, and that they had sex again there four days later.