A troubled Linda Fratianne will defend her world figure skating championship starting Thursday after having fluid drained today from her injured right ankle, which recently received an injection of cortisone.

Fratianne and her coach, Frank Carroll, made the decision to compete after a successful practice session tonight.

The three-day world chamapionship will begin here with Fratianne concerned about her ankle and what effect her former coach, Carlo Fassi, will have on the judging.

Fratianne, who plans to have surgery on the ankle when she returns to the U.S., finished second to Anett Poetzsch of East Germany in the Winter Olympics last month.

Fratianne's mother Virginia was so upset with the judging at Lake Placid, and the adverse influence she claimed Fassi had on the judges, that she said here daughter would not compete here. But after meeting with President Carter, along with other U.S. Olympians, the day after the Games, Fratianne said she would enter the competition.

"I noticed a swelling and had pain (in the right ankle) during the Olympics," Fratianne said today, soaking her right leg in a bucket of water. "A few days after the Olympics, the ankle was hurting too much to do anything and I had to have it drained.

"A week before last Friday, I had a shot of cortisone to try and keep down the swelling . . . But the cortisone hasn't worked very well. My doctor in the States wanted to operate on it even before I came over here."

The swollen area is slightly purple, and stretches the skin taut just over the inside of the ankle. It's an oval bump about 1 1/2 inches long and shows a distension of at least three-quarters of an inch. Fratianne winced each time her mother lifted her ankle out of the pail to rub on ointment.

"I really got worried about the swelling," Fratianne said . . . "because of the short and the free programs. There's a lot of stress on the foot in them, mostly because of the hard landings after a rotation."

It was in these two programs that Fratianne outscored Poetzsch at Lake Placid, only to be unable to overcome low marks in the school figures.

The chief obstacle Fratianne foresees in trying to win her third world championship -- outside of the problem with her ankle -- is Fassi. An elegant-looking, middle-aged man, Fassi was an Italian skating champion in the early 1950s, and is now a coach at various clubs in the United States. He used to be Fratianne's coach.

International skating officials are upset at Virginia Fratianne's charges against Fassi and the slight to their judges. The vice president of the International Skating Union, John Shoemaker, said Tuesday, "The charges are really unimaginable and I don't know what came over Linda's mother." Fassi also has denied all the charges.

Now, Fratianne has joined in the charges: "Everything that's held against him (Fassi) and more, is true," she said today.

"Fassi has no right to be on the USFSA (United States Figure Skating Association) Executive Committee, because he is not behind the team 100 percent," she said. "He never helps at the American team coaching or strategy; instead, he spends his time with foreign team officials, foreign skaters and, above all, foreign judges.

"What he's trying to do is get his own students, like Robin Cousins (a British citizen), on top." Cousins won the gold at Lake Placid.

"Fassi is just malicious," Fratianne said, leaning forward in her chair. "Before the nationals this year in Atlanta, he said on television that he thought Lisa-Marie Allen should take the top place. Now, Lisa wasn't even his student, so he was doing that just to try to destroy my concentration."

Fratianne missed a combination at the nationals, fell, and got up only to fall again. "At the nationals, I did lose my concentration, but that won't happen here," she insisted.

"The question is, how do I deal with the fact that Fassi might be working against me again? I try to block it out of my mind. I try not to think of it. I just keep trying to think that it'll be all right.

Fratianne said she didn't know if Fassi will "try again to affect my concentration and to influence the judges against me. I just don't know."