Fifth-seeded Gabriela Sabatini endured 2 hours 29 minutes of heavy topspin and arcing moonballs before she made the semifinals of the U.S. Open tonight over unseeded Leila Meskhi, 7-6 (7-5), 6-4. In one of the stranger straight-set matches of the tournament, their marathon included 13 service breaks and made No. 8 Mary Joe Fernandez's 6-2, 2-6, 6-1 victory over No. 9 Manuela Maleeva-Fragniere look routine.

Meskhi, ranked 21st, was the only unseeded woman in the quarterfinals, which represented her best result in a Grand Slam event. A small, 22-year-old brunette who recently married a Soviet policeman and is nicknamed "Pesky," she held two set points against Sabatini in the opening set and led by a service break in the second before the Argentine wrested control.

"It was my match," Meskhi said through her coach and translator, former Soviet star Olga Morozova.

Sabatini might have expected a much easier time, since she defeated Meskhi in the fourth round here last year by 6-2, 6-0. But they seesawed through an interminable first set that entailed six breaks of serve, endless looping rallies from the baseline, and lasted longer than many matches -- 1:20. Sabatini killed a set point as she served at 4-5, and another as she served at 5-6. She fell behind by 2-5 in the tiebreaker.

But Sabatini swept the last five points, changing tactics to rush the net, and aided considerably by Meskhi. The Soviet double-faulted to even it at 5, then jerked an overhead into the net to give a first set point to Sabatini, who rushed the net and slid a backhand volley to the corner.

The second set followed the same pattern, with seven breaks of serve. Sabatini attacked and broke a final time as Meskhi served at 30-30 in the 10th game. The Argentine rushed the net for a lunging, twisting backhand drop volley that Meskhi could not retrieve, to get to match point. Sabatini outlasted Meskhi in a topspin rally, the Soviet driving a forehand approach to the corner just deep to end it.

"I had to do something," Sabatini said. "Because she was not going to lose the points. I had to win."

The match, for all of its messiness, was closer and more absorbing than Fernandez's victory over Maleeva-Fragniere, who could not reproduce her fourth-round form when she upset Martina Navratilova, and lost a war of attrition from the baseline.

Fernandez thus proved that she is capable of big things when she is healthy, with her most significant performance since she made the Australian Open final in January, losing to Steffi Graf. The U.S. Open is Fernandez's second tournament since June. She had withdrawn or retired in three of the last six events she entered with, among other other things, tendinitis in her right shoulder, a pulled muscle in her back, and torn cartilage in her right knee.

"It's been frustrating because when I was playing, I was playing well," she said. "I was enjoying myself and getting confidence, but it was like I was starting over each time."

Fernandez broke Maleeva-Fragniere in the first and last games of the final set to make it a rout.

"I don't know what happened, exactly," Maleeva-Fragniere said. "It went very fast."

Open Notes: David Wheaton and Paul Annacone posted a doubles victory over Sergei Bruguera and Tomas Carbonell to reach the semifinals, 7-5, 6-7 (5-7), 4-6, 6-4, 6-3.

That would seem to be exactly what Wheaton did not need the day before he plays John McEnroe. But there was a plus: Wheaton had never played on Stadium Court, so he got a little early exposure before facing McEnroe. . . .

In women's doubles, Jana Novotna and Helena Sukova continue on course toward a Grand Slam. Having already won the Australian and French opens and Wimbledon, the top seeds today scored a 7-6 (9-7), 6-3 quarterfinal triumph over fifth-seeded Arantxa Sanchez Vicario and Robin White. . . .

Fourth-seeded Zina Garrison has a sore ankle. She developed an irritated Achilles' tendon in a mixed doubles match Monday. She had taped the ankle, and apparently the pressure of the bandage exacerbated the problem.

She spent much of today receiving treatment. However, she is expected to be all right for Wednesday's quarterfinal against No. 6 Sanchez Vicario.