The Washington Diplomats believe that fear will not be a factor when they play the two-time defending NASL champion Cosmos Sunday. At the same time, they also realize they must put up the battle of their lives to defeat the soccer league's most powerful club at RFK Stadium in the 2:30 p.m. contest.

"You know they are the best team in the league on paper," said Washington defender Don Droege. "In the back of your mind you feel confident, while at the same time . . .

"When I was with Rochester, I always felt like the underdog," added Droege, acquired from the Lancers this year. "I didn't have confidence in my team and that only once, in overtime, in five tries. They have great players, but they can be beat."

Washington (2-0) has not fared well against the Cosmos in the past. The Cosmos have a 6-3 advantage in games played while outscoring the Dips, 37-14, in the nine games.

For the Dips to beat the talentladen Cosmos, they must control the midfield and shut down the NASL's top scorer of a year ago, Giorgio Chinaglia.

Neither will be easy, Chinaglia, who set a NASL scoring mark with 34 goals and 79 points last year, picked up where he left off by knocking in the Cosmos' first goal of this season last week in his team's 2-1 overtime victory over San Diego.

"Teams are afraid of us," said Cosmos midfielder Terry Garbett, "and they have every right to be."

Even the Cosmos thought the San Diego players were a bit tentative, and that it cost them the game. Washington, an aggressive bunch whose physical play can be intimidating, will have to depend on its rugged defense against the Cosmos.

"We'll have to mark them tight, all over the field," said Droege, who, along with Mike Dillon, will be assigned to watch the elusive Chinaglia. "We've gone mainly with a zone-type defense and we haven't really been tested yet. There's no sense changing to a single-coverage defense now.

"They have good wing players and Tommy (O'Hara) and (Robert) Iarusci will have to keep them from turning," said Droege. "If they don't that'll be the ball game."

Stopping Chinaglia and forwards Dennis Tueart and Seninho will be only half of the Washington headache. Midfielders Franz Beckenbauer and Vladislav Bogicevic are especially offensive-minded, as their 1978 point totals of 41 and 45, respectively, prove.

The Cosmos, a patient lull-you-to-sleep team, often throw as many as seven players forward on offense. That tactic leaves them vulnerable in the back but they apply so much pressure on defenses that opponents tend to think only of survival. The Cosmos led the league in goals last year with 88, 33 more than the Dips had.

"We lost to the Cosmos twice last year and we don't intend to have that happen again," said Washington Coach Gordon Bradley. "We're off to a good start and a win over New York could be just the boost we need to really get rolling."

"A win against them would do a lot for our confidence, especially since our next two games would be on the road," said Droege. "It's too early in the season to think any game is a 'must game.' But the game means a lot to us."

Washington's plan of attack will be pressure, pressure and more pressure. Paul Cannell, the team's leading scorer last year, will be available for his first game. He and forwards Bobby Stokes, Alan Green, who scored two goals in the Dips' 5-1 trouncing of Atlanta Sunday, and Sonny Askew will test a suspect Cosmos defense.

Injuries to several Cosmos defenders sent Warner Communications, the Cosmos' owners, looking for help. They came up with a jewel in Dutch star Wim Rijsbergen, who will start at the sweeper spot. Defender of the year Carlos Alberto has been slowed by a pulled groin muscle but probably will play.

Last year the Dips defeated a Cosmos team that was without six starters, 2-1, before 17,095.

Ken Mokgojoa, the other half of the Washington scoring attack last year, is completing his season in South Africa and is expected to join the team in time for the Fort Lauderdale game April 14 . . . Cosmos Coach Eddie Firmani ranks second in the NASL in career wins with 84. Fort Lauderdale Coach Ron Newman is first with 100 . . . Firmani has won three NASL titles, two with the Cosmos and one with Tampa Bay . . . Dips midfielder Joe Horvath is tied with Philadelphia's David Robb for the lead in scoring with six points. Green has five points.