When you got off the Flushing Line subway at the Willets Point Boulevard/Shea Stadium station in Queens yesterday afternoon, blue-uniformed New York Transit Authority men with flat-topped, brimmed-hatbox caps reminiscent of Paris gendarmes were there hollering directions: "Baseball downstairs, tennis straight ahead."

The tennis yesterday was the U.S. Open championships, where Chris Evert won her fourth consecutive women's title, defeating Pam Shriver, 7-5, 6-4, and Jimmy Connors toppled Wimbledon champion Bjorn Borg, 6-4, 6-2, 6-2.

Perhaps nothing better symbolizes what has happened to the once elitist, white-flanneled sport of tennis in the last decade than the fact that the U.S. Open. America's premier tournament, is now played in a public park, a quarter of a mile from the home of baseball's New York Mets and the football Jets.

After 98 years in stuffy, private clubs - first the Newport Casino amid Rhode Island coastal mansions, and for the past 54 years at the West Side Tennis Club in Forest Hills, Queens - the national championships moved this year to the new $10 million National Tennis Center in Flushing Meadow Park, site of the 1939 and 1964 World's Fairs.

The 16-acre complex - with 19,000-seat stadium, 6,000-seat adjacent grandstand, 25 additional outdoor courts and nine indoors, all with the same rubberized asphalt surface - is open to the public year-round, and utilized 60 days a year by the U.S. Tennis Association for the Open and other events under a 15-year lease agreement with the New York Department of Parks and Recreation.

Yesterday there were only 9,256 people at the Mets-Pittsburgh Pirates baseball game, while almost 20,000 spectators overflowed Louis Armstrong Stadium, centerpiece of the tennis grounds, for the women's and men's singles finals of the $507,000 Open.

"Every time a train pulls up, about 200 people get out of each car," said conductor Edward Mckenna of the Transit Authority. "Maybe 20 of them go to Shea, and 180 to the tennis."

Little wonder, considering the attractive matchcups for the finals of the memorable two-week tournament: 1975-76-77 champion Evert versus 16-year-old Shriver of Lutherville, Md., youngest finalist in the history of the U.S. championships, for the women's title; Borg and Connors, the tennis rivals of the decade, head-to-head again for the men's crown.

Evert, 23, was too steady from the backcourt, too deadly on her passing shots, for the aggresive, net-rushing and fearless Shriver. Evert became only the second woman to win four consecutive U.S. national singles titles, matching the feat of Helen Jacobs in 1932-35.

But at 1:30 yesterday afternoon, 30 minutes before the Mets-Pirates game, there was a trickle of people entering Shea, and a stream heading for the tennis complex.

It would be 2 1/2 hours before Evert and Shriver took the court, 4 1/2 before the start of the latest battle between Borg and Connors - human thoroughbreds whose rivalry is as fierce and fascinating as that between those equine superstars, Affrimed and Alydar.

But doubles and age-group finals were already on, having started at 1 o'clock. They were the hors d'oeuvres of an all-day smorgasbord of tennis.

Acres of verdant parkland and shady groves also beckoned, and so longtime aficionados and tennis neophytes alike paraded jauntily, eagerly, toward "The Meadow."

Many carried wicker picnic baskets, ice chests, cushions - all the goodies and comforts needed for a long day of watching fuzzy yellow balls zip and float across a net. The number of plaid Madras slacks, Cucci loafers and Perrier water jugs left no doubt that these were the tennis fans, the baseball crowd having exited down the stairway.

Connecting the subway platform and parking lots with the tennis center is a 200-yard clevated boardwalk. From the first step on it, the legions who have been afflicted by the tennis epidemic were besieged by entrepreneurs - scalpers peddling $15 tickets for twice their face value, and street vendors hustling the necessities of lift: "Red-hot jumbo pretzels" . . . "Italian ices, cherry, lemon, chocolate" . . . "U.S. Open T-shirts."

The approach to the newest Mecca of international tennis is ugly, the flagpole-lined board walk loading past grimy cinder block bus barns, weed-infested parking lots, and a subway yard.

Beyond these eyesores and the cement concourse of a Long Island Railroad Station, however, is the park - a vast expanse of well-kept lawn and playing fields dotted with clusters of maples, oaks and evergreens.

To the right is the Louis Armstrong Stadium - originally built as the Singer Bowl for the '64 World's Fair, renamed for the jazz great when it became a concert arena, but neglected and decaying until the U.S. Tennis Association enlarged and refurbished it.

Planes roared overhead through the Wedgewood blue sky yesterday, making their deafening approach to nearby LaGuardia Airport, but the park nonetheless had an air of tranquility in the Sunday sun. Soccer games, volleyball and all manner of freestyle exercise were in progress. Picknickers spread out their lunches on beach blankets, and lovers strolled and necked.

In the distance, beyond the stadium, the sun glistened off the silvery globe of the Unisphere, most famous of the remaining World's Fair monuments. Children frolicked in the fountains that surround it, and marveled at the rainbows created by the man-made geysers.

The tennis center is not finished yet. It is something of a miracle that it is functional, since construction began in earnest only last April. When the landscaping is completed, promised one USTA official, "this place will be more beautiful than Wimbledon."

It already has its own charms: spacious promenades around the outside courts, yellow-and-white striped picnic tents, a corporate entertainment pavilion amid a crescent of 40-year-old sycamores called "The Grove."

Several evenings last week, spectators at the end of the matinee sessions and early arrivals for evening play were treated to post-card sunsets: a flaming ball of orange dipping beneath the stadium walls, highlighting wateroclor shades of salmon and pink, with the majestic Manhattan skyline silhouetted on the horizon like a distance and magical Oz.

This stadium is only couple of miles, as the 747s fly, from the Forest Hills section of Queens, but it is light years removed from the gabled Tudor clubhouse and starchy atmosphere of the West Side Club, where a discreetly lettered sign at the gate decreed:

"All white or off white shall be worn on court and club grounds . . . This applies to all guests as well as members." The club grounds used to be open to the public during the Open, but never the clubhouse, nor the bosom of the membership.

Commercialism, and all the trappings of corporate partonage of pro-tennis, are evident at the tennis center. A sign at the main gate to proclaims: "Planters Peanuts Welcomes You to the U.S. Open Tennis Championships." Mr. Peanut - a man in a plastic nut shell and top hat - passes out sun visors inside.

Most of the advertising is tastefull done, subdued, and the flowers around the court and walkways are real - purple and white chrysanthemums, rather than the plastic geraniums that defiled Forest Hills at Open time.

Some players, especially Europeans and South Americans bred on clay courts, have complained about the hard surface. Others dislike the noise, the bustle, the planes roaring overhead.

But most are delighted to be away from cramped and stuffy Forest Hills. "This place," said Arthur Ashe, with more enthusiasm than originality, "is the greatest thing since sliced bread."

Vitas Gerulaitis, who grew up street-wise in Brooklyn and Queens and escalated socially into the globetrotting tennis jet set, says he personally prefers the quiet, cathedral-like atmosphere of Wimbledon, the tennis shrine in suburban London. But he appreciates the effort that has gone into the National Tennis Center.

"This place will be a beautiful facility in another year. . . It's a perfectly appropriate setting for tennis in New York," he said. "Everybody was complaining that tennis was too much for the rich, a gentlemen's sport, and they wanted to change it into something as public and accessible as baseball."