The witness, an abortion patient of Dr.Robert J.Sherman's, was about to begin her testimony in D.C. Superior Court yesterday when the juror, a young woman dressed in a bright yellow pantsuit, suddenly raised her hand and announced, in a frightened voice: "I know her!"

There was a brief moment of confusion as startled prosecution and defense lawyers, just reassembled after a quiet lunch hour, looked to Judge Fred B. Ugast for guidance. Ugast then asked the witness and the other jurors to leave the courtroom.

Juror Frieda Hansborough, left alone in the jury box, in seat number three, then explained that a close relative of hers had dated the 20-year-old witness for several years.

The witness had become pregnant, Hansborough anxiously told Ugast, and she had had an abortion.

Ugast called Hansborough and the lawyers to the bench. Then defense attorneys Robert F.Muse and Constance O'Bryant conferred with Sherman, who remained seated at the defense table in the well of the courtroom.

Twenty-five minutes later, Ugast told Hansborough, who had heard almost seven days of testimony in Sherman's trial on charges of second degree murder and perjury, that he would have to excuse her from the case and replace her with one of four alternate jurors.

"Okay," Hansborough replied quietly.

Usually, such unpleasant surprises are avoided through the pratice of reading potential witness' names to the pool of jurors before the final jury panel is selected.

But Hansborough only knew the witness by her nickname, she explained to the court yesterday.

Once Hansborough left the courtroom, the jurors returned to their seats, and alternate juror number one, a middle-aged woman, took Hansborough's place in seat number three.

Then the witness, dressed in a dark pink dress with a flounced skirt and black high-heeled shoes, returned to the stand to recount her version of the Saturday morning four years ago when she went to Dr.Sherman for an abortion.

Often groping for the correct word, the witness described the procedure she underwent at Sherman's Columbia Family Planning Clinic, once located at 1835 I St. NW.

When she returned home that day, the woman testified, she was weak and in pain. The next day, after severe complications clearly indicating the abortion had been incomplete, she was taken to a hospital emergency room for treatment, she continued.

Government prosecutors contend they will prove that Sherman deliberately performed incomplete abortions on patients at his clinic, in order to necessitate their returning for follow-up visits. The prosecutors also contend that Sherman used unsterile instruments and allowed unqualified personnel to perform medical procedures in order to save money and increase profits.

Sherman, 65, is charged with second-degree murder in connection with the death of 16-year-old Rita McDowell. At the outset of the case, Principal Assistant U.S. Attorney Carl S.Rauh told the jury the government's evidence would show McDowell's death resulted from severe blood poisoning and shock that followed an incomplete abortion Sherman performed on her in March 1975.

But defense attorney Muse told the jury that he would show McDowell's death resulted from grossly negligent treatment she received at D.C.General Hospital. Sherman has pleaded innocent to all charges.

Yesterday, a former receptionist at the clinic testified that Sherman reused disposable instruments and regularly performed abortion in the clinic on women past the 12th week of pregnancy. Such abortions, according to the government's evidence, are supposed to be performed in a hospital.

A city Medical examiner also testified yesterday that Sherman received about $121 in Medical payments for treatment of Rita McDowell.