Defending champion Bjorn Borg, Vitas Gerulaitis, and Hans Gildemeister reached the quarterfinals of the French Open tennis championships today as the world's premier clay-court championship reached its halfway point at Stade Roland Garros.

Borg, who has ruled here three of the past five years, ousted the last of the native players, sturdy Gilles Moretton, 7-5, 6-4, 6-2.

Borg, seeded No. 1 and expected to meet Jimmy Connors in the final next Sunday, next will play Gildemeister, the unorthodox but tenacious Chilean Davis Cup player who outslugged American Tim Gullikson today, 6-7, 6-3, 4-6, 6-1, 6-3.

Gerulaitis, sharp at the net and steady from the back court, trounced Ivan Lendl of Czechoslovakia, the No. 1 junior player in the world last year and conqueror of Arthur Ashe on Friday, 6-2, 6-1, 6-3.

Spaniard Jose Higueras was leading 20-year-old Californian Eliot Teltscher, 6-3, 6-4, 3-2, when a heavy thundershower brought a premature end to the day's play.

The bottom half of the men's singles round of 16 will be played Monday, pairing Harold Solomon against Victor Pecci, Gene Mayer versus 1977 champion Guillermo Vilas, Eiddie Dibbs meeting Wojtek Fibak and Manuel Orantes vying with Connors.

In the women's singles, Australian Wendy Turnbull and Czechoslovakians Hana Mandlikova, Regina Marsikova and Renata Tomanova reached the quarterfinals today, prompting an English press-box wit to remark that "the bottom half of the ladies' draw looks like the same thing ova and ova."

Turnbull ousted California Leslie Allen, 6-2, 6-3. Mandlikova, a richly promising junior, eliminated Betty Ann Stuart, 6-4, 7-6. Marsikova's passing shots were too good for the aggressive thrusts of Texan Anne Smith, 6-3, 7-6, and Tomanova defeated Romanian Lucia Romanov, 6-4, 6-2.

The women's quarterfinals pairings are Chris Evert against Ruta Gerulaitis, Australian Dianne Fromholtz meeting defending champ Virginia Ruzici of Romania, Turnbull facing Mandlikova and Marsikova versus Tomanova.

Borg, who started the week sluggishly, was in good form again against Moretton, a tall, muscular 20-year-old who bears a stricking resemblance to heavyweight fighter Joe Bugner.

Moretton has a powerful serve and forehand, but Borg played mostly to his more defensive backhand after the first set, and warded off a storm of overhand volleys with some outstanding passing shots.

"I'm feeling much better every day, in my legs and in my strokes," said the champion. "I'm moving well and hitting the ball well now. I feel very good."

Gerulaitis also was impressive, more so to Lendl than to anyone else. "I knew before the match that he was very fast, but I didn't believe he was that fast," said the promising young Czech, who was intimidated by Gerulaitis' quickness and unerring net play.