Pieces

In 1942, a 10-year-old named Timmy plays with a jigsaw puzzle of a nude woman when his mother walks in and chastises him for it. After she orders him to get a plastic bag, he returns with an axe and murders her, then saws her body with a hacksaw. When the police arrive, they find him hidding inside a closet and Timmy acts as if he was a witness of the crime scene. The police believe this, not suspecting him as the murderer, and is sent to live with his aunt. Forty years later, an black-clad figure opens a box containing the bloodied clothing and a photograph of Timmy's Mother. He also unboxes the bloodied jigsaw puzzle and starts to put it together.
Meanwhile, as a nearby girl studies outside in broad daylight, she is decapitated with a chainsaw by the unidentified killer and steals her head just before Lt. Bracken and his partner Sgt. Holden arrive to investigate the murder. After the detectives explain to the college's Dean that there were no witnesses or suspect, the Dean asks the rather sinister Professor Brown (Jack Taylor), who teaches anatomy, to show the detectives around. Outside, the groundskeeper named Willard (Paul Smith) is seen trimming a hedge with a chainsaw, similar to the killer's. In the campus' library, a student named Kendall receives a note given by a girl to come to the pool later; the killer finds it and tracks down the girl at the swimming pool, where she is brutally sawed with the chainsaw. A little later, Willard arrives on the scene and is arrested, believing to be a suspect. Near the pool, they find the chainsaw and the girl's body parts, with the exception being her torso is missing.
The next day, Dr. Jennings meets with Kendall at the station in hopes that Kendall can help provide a profile of the murderer. Lt. Bracken brings in an undercover cop named Mary Riggs, who was also a former tennis player. Bracken explains to Holden that Mary is going undercover to pose as a tennis instructor at the college, and Kendall is going to assist her whenever he can. As a reporter named Sylvia Costa is stonewalled by Bracken, the killer stalks a girl later that evening, who had finished her dance routine, and saws her arms off inside an elevator just before Kendall and the police arrive. During the same evening, the killer also stalks Sylvia and stabs her on a waterbed.
Later the next day, one of Mary's tennis students heads into the locker room after the killer turns on music on the loudspeakers, and is pursued until she meets her demise by having her waist sawed horizontally. While Mary and Kendall focus on turning off the loudspeaker's music, the killer steals the girl's legs and escapes. Prior to Mary getting enraged for the killer's escape, her and Kendall find Willard being released from custody due to lack of evidence. Kendall then goes to the police station and presents his theory to Holden about the killer being part of the university faculty, since the killer knows when and where to strike before avoiding the police. They spend hours researching files on the faculty and discover that the Dean previously changed his name and that his mother was brutally murdered, finding out the Dean's identity to be a grown-up Timmy. Meanwhile, Mary is drugged by the Dean at his apartment and attempts to saw off her feet after the previous victim's feet didn't fit his mother's shoes for the puzzle. Bracken, Holden, and Kendall burst into the Dean's apartment, and the Dean gets shot in the head by Bracken while he was trying to kill Kendall. After this, Kendall leans on a bookshelf which switches around and contains the Dean's human puzzle; a decomposing body of the Dean's victim's body parts stitched together. The stitched jigsaw corpse falls on Kendall, and horrifies him. Later on, a shaken Kendall goes with Holden and just as he grabs his jacket, the jigsaw-corpse inexplicably comes to life and castrates him as he screams.

Question: What profession does Mary Briggs impersonate when undercover?
Answer: tennis instructor