John McEnroie, the defending champion, earned an impressive 6-2, 7-5 quarterfinal victory over Hank Pfister in the $125,000 Queens Club grass-court tennis championships in London yesterday, but couldn't do it without a temper tantrum.

McEnroe was given a conduct warning by umpire Ian Stirk for using "an obscenity" during a noisy argument with Stirk in the second set.

McEnroe clinched a place in Saturday's semifinal against Brian Teacher, who ousted Peter Rennert.

The other semifinal will be between Brian Gottfried and John Sadri, who defeated Eddie Edwards and Kevin Curren, respectively.

McEnroe became upset early in the match over the noise from the members' restaurant and finally let go in the third game of the second set after one of several close line calls.

He walked toward Stirk, asked him if he agreed with the lineman's decision and, apparently getting an answer he didn't like, made a remark to Stirk.

That earned him the conduct warning, and when he asked the reason for it he was told, "for using an audible obscenity."

McEnroe replied, "I'll tell you an obscene word when I tell you how I think you umpired this match. You make amateurs look like professionals."