The sex-for-grades corruption trial against law professor Tey Tsun Hang was adjourned, after he suddenly slumped over while in the dock.

The hearing had just resumed after lunch.

Lawyer Peter Low, who is advising Tey as he conducted his own defence, told the court that his client was "hyperventilating".

Chief District Judge Tan Siong Thye allowed a brief adjournment until 3.30pm.

Tey who is on trial for corruptly obtaining gifts and sex from a 23-year-old student in exchange for improving her grades, said in court earlier today that a senior anti-graft investigator made a death threat against him.

Tey had earlier decided not to continue his cross-examination Mr Teng Khee Fatt from the Corrupt Practices Investigation Bureau (CPIB).

He told the court: "Your honour, I don't feel quite well. I have no more questions for cross-examination."

Two female lawyers rushed to his aid during the break, with a damp cloth and medicated oil. They also helped Tey remove his lawyer's robe, which he had been wearing since the start of the trial earlier this month.

Tey remained seated in the dock with a handkerchief over his face, taking deep breaths into a plastic bag.

This is the ninth and last day of the trial. The trial resumes on April 1.

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.