Defending champion Chris Evert Lloyd lost an unsettling first set to teen-ager Hana Mandlikova today, then bore down and bounced the talented young Czech, 6-7, 6-2, 6-2, to reach the final of the French Open tennis championships.

Evert, who has lost only one match on a clay court since the summer of 1973, will go for her fourth singles title here Saturday against Virginia Ruzici, who defeated Dianne Fromholtz, 7-6, 6-1, this afternoon at sundrenched Stade Roland Garros.

Evert will be a heavy favorite in the final since she has a 9-0 career record against Ruzici, her most recent victory coming last month in the final of the Italian Open at Perugia. Evert has won 33 consecutive matches on clay, 158 of her last 159, the only setback coming against Tracy Austin in the 1979 Italian Open.

However, Ruzici is a far better player than she was two years ago, when she won the French title over a weak field that was missing Evert and most of the other top women stars, who were then playing World Team Tennis in the United States.

"Then I won with my forehand and my legs," said the lithe, olive-skinned Ruzici, 25. "In the last two years, I have been working very hard with my coach (former Roamnian Davis Cup captain Stefan Georgescu). I have improved very much my serve and my backhand.

"I've been working a lot, jogging a lot, so physically I am well-prepared. And I have been thinking about Roland Garros the whole year . . . . Every time I go to bed, I think of myself winning Roland Garros instead of dreaming about something else. For us Romanians, the French is a very big title because it is on clay, the surface we grew up on, and I think I have more chance on this than any other surface."

The women have Friday off as Jimmy Connors plays Vitas Gerulaitis and defending champion BJORN dorg faces Harold Solomon in the men's singles semifinals.

Evert needed more than two hours to overcome the 18-year-old Mandlikova, who has great athletic gifts but does not yet possess steadiness or champion's mentality.

Mandlikova is tall and skinny, her straight brown hair tied back by a red headband. She has a strawberry complexion and austere but appealingly earnest features. She is an aggressive, determined player, but she plays brilliantly in patches, mindlessly in others.

Mandlikova's bursts of inspiration and mental lapses dictated the twisting pattern of the first set. Evert saved two set points on her serve at 4-5, served for the set at 6-5, then lost it in a tie breaker, 8 points to 6, after overcoming a 3-5 deficit and having a set point of her own at 6 points to 5.

Mandlikova led, 3-0, in the set, smartly changing pace and spin, attacking Evert's second serves, getting to the net behind clever approaches and angled dinks short to Evert's backhand that jerked her out of position and opened up the court.

But eventually, Evert started covering those short, soft balls, hitting winners off them, and Mandlikova became erratic on her ground strokes, particularly the backhand that she slices most of the time. She has so many shots, she sometimes doesn't seem to know which to call on, and seems to have little concept of percentage play.

In the best-of-12-point tie breaker, Mandlikova led, 4-1, and 5-3, then doubled-faulted and netted an easy smash. Evert got to set point at 6-5, but Mandlikova won the next point with a crunching overhead and an ace, then blasted a backhand, cross-court passing shot.

Evert should have started serving in the second set, but the umpire made a mistake, Mandlikova served, and neither player caught the error. Mandlikova saved one break point at 15-40 with another ace, then dumped an unforced error into the net.

From then on, Evert was never in trouble. She lost only seven points on her serve in the second set, and none until she was 4-0 up in the third.

Ruzici had lost four of five previous meetings against left-hander Fromholtz, but today she set the pace and tempo of the rallies, primarily with her thumping topspin forehand. Her backhand has improved, but it is still quite defensive, and she runs around it when she can, using the forehand to force errors, whip clean winners or pave her way to the net.

The crux of the match came with Ruzici serving at 3-4, 0-30 in the first set. She served an ace, and was off on an eight-point binge. She also played a strong game at 5-6, and won the tie breaker 7 points to 1, the only losing point being double fault.

Fromholtz played an awful tie breaker, netting forehands on the first two points, butchering a sitter into the net for 0-4.