NEW YORK (AP) _ Second-seeded Monica Seles and No. 4 Conchita Martinez battled to grab the last two semifinal spots in the U.S. Open women's singles.

After Seles scored a hard-fought 7-6 (7-5), 6-2 victory over fifth-seeded Jana Novotna today, Martinez beat back a suprising bid by No. 14 Brenda Schultz-McCarthy of the Netherlands 3-6, 7-6 (7-3), 6-2.

The two winners meet Friday, while the other semifinal matches top-seeded Steffi Graf against No. 9 Gabriela Sabatini.

Of the four semifinalists, only Martinez has not won a title on the hardcourts of the National Tennis Center, her only Grand Slam tournament crown coming on the grass at Wimbledon in 1994.

Against the big serves of Schultz-McCarthy _ she had 11 aces _ Martinez had no answer until midway through the second set.

``In the first set I was so much up, she couldn't come back,'' said Schultz-McCarthy, who was making her debut on Louis Armstrong Stadium court. ``I fought hard to get to a tiebreaker, but I should have finished it right there.''

Since she didn't, her Spanish opponent did. And after leveling the match at a set apiece, Martinez raced through the third set and into the semifinals.

Still, Schultz-McCarthy, a surprise quarterfinalist, was happy with her U.S. Open.

``I guess I really grew in this tournament,'' she said.

After Seles began the day with her victory, she made a triumphant tour of Louis Armstrong Stadium, signing autographs for fans who had flocked to courtside.

It was the toughest match Seles has had since returning to tennis following two-year break she took after being stabbed. But she was up to the task.

``She took a long time to come back, but when she came back she was ready,'' said Novotna, who has developed a habit of being on the losing end of important matches.

Until today, Seles had had her serve broken only four times in her nine matches since her comeback began, and only twice in this, the year's final Grand Slam tournament. Novotna broke her three times in the first set, in the third, fifth and 11th games.

``I played a very good first set,'' Novotna said. ``I was up 4-2. I really thought I had a good chance today because Monica wasn't serving that well. She was always serving to my backhand, which is to my advantage.''

But Novotna failed to take advantage. Seles, of course, had a lot to do with that.

The only American left in the women's field _ albeit a naturalized one _ Seles staved off two set points in the 12th game, breaking Novotna's service to send the set into a tiebreak, which she controlled.

``Monica became much more confident after winning the first set,'' Novotna admitted. ``When she needed it, she always came up with a good shot.''

In the men's singles, the U.S. Open has a decidedly red-white-and-blue tinge _ with a touch of Black.

Pete Sampras, Michael Chang and Jim Courier were victorious Tuesday to join fellow Americans Andre Agassi and Patrick McEnroe in the quarterfinals.

The only non-American in the bottom half of the draw, Zimbabwe's Byron Black, was an all-American at Southern Cal, where he won the NCAA doubles title in 1989. He reached the quarters by upsetting eighth-seeded Michael Stich, last year's runnerup to Agassi, 6-4, 6-4, 3-6, 2-6, 6-3.

Sampras, seeded second, ousted No. 15 Todd Martin 7-6 (7-4), 6-3, 6-4; Chang stopped Australian Michael Tebbutt 6-2, 6-2, 4-6, 6-3; and Courier, No. 14, eliminated No. 3 Thomas Muster 6-3, 6-0, 7-6 (7-4).

An all-American semifinal is possible if not probable. Today's quarterfinal matchups pit Agassi against Petr Korda of the Czech Republic and McEnroe against fourth-seeded Boris Becker of Germany.

On Thursday, Chang will play Courier and Sampras will take on Black.

Top-seeded Steffi Graf and No. 9 Gabriela Sabatini grabbed spots in the women's semifinals on Tuesday, Graf downing Amy Frazier 6-2, 6-3 and Sabatini defeating No. 14 Mary Joe Fernandez 6-1, 6-1 two days after Fernandez eliminated defending champion Arantxa Sanchez Vicario.

``Andre and Pete are the favorites, but with guys like Korda and Becker and Chang and Courier, I mean all of them have a good chance'' to win the Open, Martin said following his loss. ``But I would have to say that Andre and Pete would have the best chance.''

Sampras matched big serves with Martin and came out on top.

``We were both a little bit tentative in the beginning, not playing that great,'' Sampras said. ``Whenever I play Todd, it always comes down to a point here or there, and it came down to a tiebreaker and I played pretty solid.

``From that point on, I kind of escalated my tennis to a higher level and Todd, I think, got a little bit discouraged.''

Sampras finished with 18 aces, Martin 11 in a match dominated by the serve.

``Todd has got a huge serve,'' Sampras said. ``He doesn't hit it that hard, but he hits the corners, hits all the corners, and it's a very tough serve to read because he is so tall.''

In Black, Sampras will face a 5-foot-9 opponent who hits two-handed from both sides and whose only tournament title as a pro came in a challenger event in Bombay, India, in May.

``He beat Stich today, who plays just like me,'' Sampras said. ``He returns extremely well. ... I just need to stay patient.''

Muster, the king of clay, didn't stick around long enough to get patient in his match against Courier. The loss ended the Austrian's bid to become the only player to reach the U.S. Open men's singles quarterfinals in each of the last three years.

Courier pounded the ball deep and hard, pinning Muster back of the baseline and never giving him a chance to get into the match.

``It is one match, and I am very happy that I am playing well, but I don't want to make more of it than it really is,'' Courier said. ``I have got to keep my feet on the ground and keep moving because this tournament is getting nothing but harder.''