"Everybody wants to know about this mystery racket." Mike Fishback said with a grin Friday, holding aloft the bizarre weapon - a conventional metal frame with a radical string job - with which he had beaten a sluggish Stan Smith, 6-0, 6-2, in the second round of the U.S. Open tennis championships.

Fishbach, an engaging 22-year-old Long Islander, lost today to Englishman John Feaver, 2-6, 6-4, 6-0, but spectators swarmed around court 14 at the West Side Tennis Club to catch a glimpse of the racket that has caused such a furor in Europe that it was banned last week from tournaments sanctioned by the West German Tennis Federation.

Fishbach's racket is his adaptation of a design originated two years ago by a West German gardener named Werner Fischer, and first used in a major tournamentby Australian Barry Phillips-Moore in this year's French Open.

Fishbach strings his own with two sets of nylon strings, which are interwoven with cross supports of fish test line, adhesive tape, rope and a plastic, protective material called "Sapghetti."

Fishbach will not reveal the tension of his couble-string job - "I spent 30 hours doing the first one, four hours on each one now, so I'd rather keep it to myself a little bit" - but it is very loose, probably about 35 pounds.

The power comes from a trampoline effect: he ball sinks deep into the double layer of strings and is propelled forcefully by the elastic material between the strings.

"It's got 16 up-and-downs on both sides, which sandwich six rows of crosses. I've never counted the strings - I quess there are 40 or 45 in all," Fishback said. "It's a joke to look at . . . the strangest-looking thing I've ever seen."

Fishbach, who quit tennis for six months last year to commune with nature in the Green Mountains of Vermont, was fascinated with Phillips-Moore's racket in Paris.

"He'd show it to me for 30 seconds and then grab it away, saying."I want to be the only one using it'," Fishbach says. "But I ran into a guy in a photographic shop in Gstaad. Switzerland, who had one like it, I looked at it for about three minutes, ran back to the hotel, drew a diagram, came back to the States and just started stringing."