Jack Kramer, the Wimbledon champion of 1947-48, noted in a recent book would always be a formidable clay court player but vulnerable on fast surfaces such as Wimbledon grass.

That opinion was shared by many tennis experts until Borg won Wimbledon for the first time in 1976, at age 20, withoug losing a set.

"He made two adjustments in particular that impressed me with his adaptability and his dangerousness on all surfaces," wrote Kramer of that year.

"He flattened out his stroking motion somewhat, reducing that extreme vertical lift of his, and so got better depth off the ground. And he improved his serve when he had to."

Borg probably never has served better than he did in winning his third consecutive Wimbledon title this week. He put in 46 of 85 first serves in clobbering Jimmy Connors, 6-2, 6-2, 6-3, in yesterday's final, and won many points outright on his serve when he needed then most.

he thought the most important game of the match, for instance, came when he served at 2-1 in the second set. He was down 0-40, but got back to deuce after crunching three first serves and ultimately escaped one more break point to win the game. That kept him on top, and he stomped Connors from there.

"Before Wimbledon in 1976 I was practicing my serve a lot. Before that it was not a strong part of my game, but I was practicing it every single day," Borg recalled yesterday. "My coach, Lennart Bergelin, told me a few little things I was doing wrong with my feet just before I threw up the ball to serve. When I corrected these I found my rhythm, and then suddenly I was serving well."

Borg, 22, is slender - 5-foot-11 and 165 pounds - but the muscles of his broad shoulders ripple underneath his form-fitting Italian tennis shirts. He has powerful legs as well - the build, strength and swiftness of a football running back.

The topspin ground strokes he hits with such a big, roundhouse flourish - the whippy forehand and the two-fisted backhand that starts at his knees arcs past his head on the follow-through - are still the most reliable weapons in his arsenal.

But Borg has become a complete player, able to serve volley and hit slice approach shots as well. He has no obvious weaknesses any more, on any surface. Connors, in a rather defiant postmatch press conference, said he say nothing new in Borg's game.

"He plays the same all the time. He has no variation in his game at all . . He was serving pretty well, but he wasn't overpowering me in any way," said the vanquished.

Top Seeds Bow

The second-seeded team of Merrily Krauser and Dr. Ray Lake rallied from near-elimination in the second set to capture a 4-6, 7-6, 6-0 triumph over Anne and Mark Geier yesterday for the mixed doubles championship of the D.C. Public Parks tennis tournament at Edgemoor Tennis and Swin Club in Bethesda.

Leading in the second set, 6-5, the top-seeded Geiers were forced to replay a point that would have given let call. Krauser and Lake then broke captured the tie breaker, 5-4, and easily won the final set, 6-0.

The men's doubles championship was postponed until early August after Rusty Addie broke his wrist in a semifinal win last week with teammate Randy Sherfy. Addie and Sherfy, second-seeded in men's doubles, will face Lake and Fred Farzanegan for the title