In the current world tennis rankings, Guillermo Vilas is No. 6. But when the $175,000 Washington Star International Tennis Tournament begins first-round play Monday, Vilas will be the No. 1 seed.

The reason is simple: The top five players in the world - Bjorn Borg, Jimmy Connors, John Mc Enroe, Vitas Gerulaitis and Roscoe Tanner - are bypassing the tournament at the 16th and Kennedy Street courts.

That leaves Vilas, the Argentinian left-hander who dominated tennis two summers ago, as the top seed. Vilas has not played well this summer since losing a grueling Italian Open final to Gerulaitis. He was knocked out of the French Open early, lost in the second round at Wimbledon and last week lost in the first round in a tournament in Switzerland.

But he is still the top name in this tournament. Seeded directly behind Vilas are Eddie Dibbs, who lost the 1978 Star final to Connors, and Harold Solomon.

Behind them are Spanish clay-court specialists Jose Higueras and Manuel Orantes; Argentina's No. 2 player, Jose Clerc; Brian Gottfried; French Open finalist Victor Pecci, and Wimbledon semifinalist Pat DuPre.

In spite of the absence of the world's top five and such players as Arthur Ashe and Stan Smith, there is an interesting mixture of good players and young rising competitors in the 64-man draw.

Among the more intriguing entries in the tournament are:

Ben Testerman, a 17-year-old American and the youngest player in the tournament. Testerman played mixed doubles with Billie Jean King at Wimbledon. He will face DuPre in the first round in one of the more competitive early matches of the tournament.

Mel Purcell, 19, who won the Pan American Games singles title in San Juan Friday. Purcell will face Jean Joubert in the opening round and, if he and Testerman both win their opening matches, they will play in the second round.

Marty Riessen, now 37, seeded 15th, figures to meet Vilas in the third round.

Raul Ramirez, the Mexican once ranked in the top five in the world but only seeded 13th here after an extended slump for almost two years.

In many ways, this is a tournament that offers several newer players on the circuit a chance to make their mark and also slumping players like Vilas, Gottfried, Dick Stockton and Ramirez a chance to stop their slides.

The first round will be played Monday and Tuesday, with the times of the matches being decided tonight. Some of the better first-round matches include Dibbs against Terry Moor, a good clay-court player; Phil Dent against Colombia's Ivan Molina; Stockton against Kim Warwick; Gottfried against Geoff Masters; Ramirez against Ross Case, and Higueras against John Sadri.

Dibbs and Solomon both come into the tournament after good weeks at Forest Hills, Dibbs having beaten Pecci in the semifinals yesterday.

But on the slow clay here, the winner may come from the second trio ofseeds - Higueras, who reached the semis last year, Orantes and Clerc.

There will be afternoon and evening sessions Monday through Saturday, with the finals in singles and doubles scheduled for a 7 p.m. start next Sunday.