John McEnroe, upset over a new requirement for playing in the Davis Cup, said he might not represent the United States this year.

McEnroe said he was disturbed by the U.S. Tennis Association's demand that all members of the U.S. team sign a code of conduct.

"I don't see why I should have to sign it . . . simply because they were backed into a corner by some sponsor who wasn't even involved the first six years that I played," he said.

After the U.S. squad, with McEnroe, lost to Sweden in the Davis Cup final in December, Harry Merlo, president of Louisiana Pacific Corp., sponsor of the U.S. team, wrote a letter criticizing its behavior and demanded that in the future every player sign a code of conduct . . .

Pat Cash, Sammy Giammalva and Joakim Nystrom won matches in the $267,000 Belgian Indoor Championships in Brussels, joining Stefan Edberg, Anders Jarryd and Heinz Gunthardt in the quarterfinals.

Cash, seeded fourth, defeated Matt Mitchell, 7-5, 6-4. Nystrom, the sixth seed, needed more than two hours to beat John Lloyd, 4-6, 7-6 (7-4), 6-4.

Giammalva beat Chip Hooper, 7-6 (7-4), 6-1, to earn a quarterfinal match against Edberg. The second-seeded Jarryd scored a 6-3, 6-1 victory over Tomas Hogstedt and Gunthardt rallied to beat Vijay Amritraj, 1-6, 6-3, 6-0 . . .

Second-seeded Chris Evert Lloyd easily defeated Kathy Jordan, 6-2, 6-2, and No. 3 seed Helena Sukova beat Melissa Gurney, 6-2, 6-0, to advance to the quarterfinals of the $150,000 Virginia Slims tournament in Dallas.

Other quarterfinalists include: top-seeded Martina Navratilova, No. 6 Claudia Kohde-Kilsch and unseeded players Myriam Schropp and Catarina Lindqvist.