Law professor Tey Tsun Hang had revealed confidential information about a female student's results and class ranking to her over lunch, in a bid to demonstrate his power and influence over her academic performance.

The student, Ms Darinne Ko Wen Hui, then 21, was said to have given him gifts and also had sex with him twice, the court heard on Thursday as the prosecution outlined its case against Tey.

"It is the prosecution's case that (Tey's) receipt of the gifts from Ko and his sexual intercourse with her were transactions tainted with an objectively corrupt element," said the prosecution in its opening statement on day one of the sex-for-grades trial.

Tey, 41, is facing six charges of corruptly obtaining sex and gifts from Ms Ko between May and July 2010, in exchange for better grades, while she was his student at the National University of Singapore (NUS).

Ms Ko, now 23, took the witness stand on Thursday afternoon and testified that Tey had, sometime in May 2010, told her over lunch that her class ranking was 17th that year.

This information, she said, was not something she was aware of or could have found out on her own.

The lunch was also initiated by Tey and was one of several private meetings between the two to follow.

The court also heard that the two met in January 2010 when Ms Ko was a student in Tey's Equity and Trust class. She started assisting Tey in carrying out research for his book on the same subject in April that same year.

Ms Ko also testified that she had given him a $740 Montblanc pen as a belated birthday gift because she "liked" Tey "as a friend".

She is also said to have lost her virginity to Tey when she first had sex with him in his office at NUS in July 2010. She would have sex with him again within days.

The trial is expected to last until Jan 22, and the prosecution has lined up 14 witnesses, including Ms Ko.