AT SUNDOWN Friday the top tennis players in the Middle Atlantic area will begin a round-robin shootout to determine who is best. If all works according to plan El Motaz Sonbol and Gene Russo, who have battled lustily for the right to be called No.1, will slug it out in the final Monday.

Until now, rankings for this region have based on rough criteria. The shootout should end all rancor and speculation, at least for awhile.

Matches will be at the Washingotn Tennis Center in McLean on this schedule: Friday - 4 p.m. to 10 p.m.; Saturday and Sunday - two sessions each day at 1 p.m. and 5 p.m.; Monday - final match at 7 p.m. Tickets are $12 for the entire series or $3 each session.

Rating tennis players on local levels has always been complicated. COmpetitors often don't enter the same tournaments or share common opponents. Players with serve-and-volley games excel lon fast surfaces and stick to matches on cement and grass; those who revel in long baclcourt rallies enter tourneys played on slower surfaces such as clay or Har-Tru.

Last year these ranking difficulties caused a furor here.

In 1974 and '75 Russo, fomerly the third-ranked junior in Australia, topped the Middle Atlantic Lawn Tennis Association rankings in songles. Few serious challengers to his crown emerged until Sonbol, a member of the Egyptian Davis Cup Team for 15 years, came to town as the first secretary of the Egyptian Embassy's commercial office.

The two men are good friends and practice partners, but in 1976, when the tentative MALTA rankings listed Sonbol first and Russo second, Russo fired off a protest letter to the MALTA ranking committee and soon afterward the rankings were reversed.

According to lthe stocky Russo, "Every year rankings are published, and whoever is not satisfied has a chance to write a letter and complain." Russo said the uanking committee counted a tournament Sonbol won whcih was not within the proper ranking period.

Excising that tourney "cut El Motaz' record down to three tournaments in which he was a winner, a semifinalist and a quarterfinalist. I have more respect for him than anyone in this area, but that is not a particularly great record," said Russo.

Russo played in six tournaments, won three and lost to Sonbol in the final of their only head-to-head confrontation in the MALTA outdoor championships held in Hampton, Va., on a slow surface.

According to the lefthanded, fast-talking Russo, the Middle Atlantic ranking system " did not benefit the people who played more tournaments, which in any ranking system makes sense. You don't penalize guys for playing more tournaments, and I played more tournaments than anyone.

Sombol, however, did win their only confrontation and conquered the field in the unsanctioned D.C. Public Parks Tournament, which is not included by MALTA in its determination of rankings.

The trim, 40-year-old Sonbol, who measures words with as much care as he uses on his ground strokes, has another story. "I have a direct victory (over Russo). Maybe he played more tournaments than me, but the number of tournaments required by MALTA is three, not more. I played in four. I understand they count the direct victories and the quality of the players who play in each tournament, not how many tournaments you played in."

There was only one way to resolve the controversy - a round-robin tournament including Russo, Sonbol and other leading area players. In round-robin events, each player must play everyone in his division; no one is eliminated after one loss.

And now the round-robin is upon us, dubbed the Greater Washington Tennis Classic for the Yonex Challenge Cup and intended as an annual event. The eight participants will compete for prize money, a first in area tennis competition.

There can be no excuses. The matches will be held on a "supreme" court surface, the surface used by the World Championship of Tennis (WCT) touring pros. It is considered neutral ground, neither slow nor fast.

Hopefully, Russo and Sonbol will meet in the tournament final and give fans a chance to judge who is best. Their match should showcase the two prevailing approaches to tennis.

The 31-year-old Russo wallops his serve, moves in on weak returns and volleys for winners.He grew up playing on the Australian grass courts, which sired finesse sluggers like Joh Newcombe.

Sonbol learned to play on the show clay courts in Egypt. He trains hard, has good speed and endurance and is a master of the controlled, patient back court game. His strategy is to keep the ball in play until his opponent forces a shot and makes an error. Consistency is his weapon against Russo.

Russo is 10 pounds overweight and admits his court movement and speed need work. The key to his game is his powerful serve. After blasting one of these cannonballs Russo moves to the net. He snaps his wrists on his slashing volleys, a style that is rare among American pros.

Sonbol sums up the secret to his game in one word: practice. He does not work on one stroke. "I work on everything."

Who is the top tennis player in these parts? The Monday night final should determine whether Russo, Sonbol or one of the several talented dark horses in the field can claim the crown.

Other entries are Bobby Goeltz, Steve Fisk, Steve Tidball, Fred Drilling and Scott Kid. The eighth competitor will be chosen from 16 entrants playing in the opening night qualifying round Friday.