The Cosmos outscored the Washington Diplomats, 3-2, in a pulsating sudden-death tie-breaker shootout yesterday and escaped with a dramatic 3-2 decision in a North American Soccer League battle before 24,862 at RFK Stadium.

Seninho scored the Cosmos' winning goal in the shootout with a waist-high shot into the left-hand corner of the net. The Dips' tying attempt by Mike Dillon was saved by goalie Erol Yasin, and the Cosmos left town a winner.

The third-largest crowd to witness a Dip game at RFK was deeply disappointed after watching the hustling home forces battle back to tie the contest at 2-2 on two Alan Green goals, only to miss seven of nine shootout tries and suffer their first loss of the year.

Washington, now 2-1, still remained in first place ahead of the Cosmos in the National Conference East Division with 19 points. The Cosmos (2-0) picked up the maximum nine points (six for a victory and one point for each goal up to a maximum of three), and are second with 17.

The Cosmos, outplayed much of the contest by the spirited Dips, surrendered the two Green goals within a 93-second span before forcing the game into overtime.

Neither team scored in the two 7 1/2 minute overtime sessions and the teams went into the tie breaker. Each team alternates five players who start with the ball on the 35-yard line. They go against the goalie one on one, having five seconds to attempt to score.

Yasin and Washington goalie Bill Irwin were nothing less than brilliant in the shootout, stopping many attempts at point-blank range.

Irwin opened the shootout with a sparkling sliding block of Giorgio Chinaglia's shot. Washington's Sakib Viteskic chipped his attempt just over Yasin's leap into the net and the Dips took a 1-0 advantage.

Irwin then forced misses by Dennis Tueart and Marinho sandwiched around a Franz Beckenbauer goal while Yasin successfully defended shots by Tommy O'Hara, Sonny Askew and Don Droege.

Carlos Alberto, the NASL Defender of the Year last season, then put the Cosmos ahead, 2-1, on a shot that just eluded Irwin's dive.

That put the pressure on Green. The quick English forward dribbled straight at the fast-approaching Yasin, made a quick move to his right and lofted the ball into the left side of the net to send the game into a sudden death shootout.

"You try to stay in the same spot until the man makes his move." said Yasin, who was credited with four saves and made at least a dozen interceptions of Washington passes. "You don't have much time so you almost have to try the same thing on each man. He (Green) went around me."

Washington then missed its golden opportunity to upset the two-time defending league champions. Cosmos Ricky Davis, Santiago Formoso and Wim Rijsbergen each misfired or was throttled by Irwin on their attempts.

But Washington, needing only to make one shot after a Cosmos miss, was foiled as Yesin forced Joe Horvath to miss wide left, blocked Robert Iarusci's shot and deflected Paul Cannell's try just enough to send the game into the fourth round of sudden death.

"I missed it by a few inches," said Cannell, who replaced Bobby Stokes in the second overtime period "It was my first action and I wasn't really that fit. I didn't expect to play much. But for 7 1/2 minutes I could go all out."

Seninho, the fleet winger who made enough long runs against a determined Diplomat defense to qualify for a marathon, then put the pressure on Washington again with a slick goal that flew past Irwin.

"Players try different things," said Irwin, who made four saves. "Usually I want the players to go wide because it cuts down on the time they have to shoot. He must have got it in just in time."

Dip defender Dillon's tying attempt was then swallowed up by Yasin and the Cosmos were safe.

"A shootout is anyone's game," said Washington Coach Gordon Bradley. "We did what we intended to do. We carried the game to them. We were not overawed by them and they were the tired team after 90 minutes."

Washington outshot the Cosmos, 21-17, and had its share of early opportunities. But on more than one occasion, Alberto, Beckenbauer, Rijsbergen, and Yasin were in the right spot to stymie Washington threats.

Tueart's 12-foot hook shot following a corner kick put the Cosmos ahead, 1-0, with just 8:13 elapsed in the first half.

The highest-scoring team in the NASL last year stung the Diplomats quickly in the second half. Chinaglia, the league's top scorer in 1978 with 79 points, took advantage of a Washington lapse on defense and zipped in a goal from 18-yards out.

Green then took matters in his own hands. He intercepted a ball in the Cosmos goal area and scored unassisted from 10 yards out to cut the deficit to 2-1 at the 47:44 mark.

Before the Cosmos had time to think about that glaring error, Green had tied the game at 2-2 on a 15-yard header following assists from Iarusci and Askew.

"We didn't have a chance to think we were out of it," said Droege. "We scored two goals so quick."

"That first goal was a defensive mistake," said Tueart. "The second one he earned. We're coasting, 2-0, and try to give the game away."

The first overtime session was all defense. But in the second session, both teams went on the attack.

The Dips came up with two defensive gems to keep the Cosmos from scoring. Irwin gambled on a play that scared both his coach and Cosmos' winger Seninho.

A long pass was sent in the direction of all-alone Seninho and Chinaglia near the 35-yard line. Irwin, who said later he thought he could beat both men to the ball, left the net unprotected and charged for the ball. Seninho froze and Irwin managed to boot the ball out of bounds. CAPTION: Picture 1, Alan Green, who had both goals for Diplomats, dribbles around newest Cosmos player, Wim Rijsbergen of Holland. By Richard Darcey-The Washington Post; Picture 2, Cosmos goalie Erol Yasin, right, prepares to grab a chip shot by Sonny Askew (17) of the Diplomats. Yasin made some spectacular stops as Cosmos won sudden-death shootout, 3-2. By Richard Darcey-The Washington Post