In what he called "probably the most gratifying experience of my career," Elvin Hayes almost single-handedly lifted the Washington Bullets to a stunning overtime victory against the heavily favored Philadelphia 76ers yesterday.

Hayes scored nine of the Bullets' 13 overtime points - including their last seven - as Washington pulled away to a 122-117 triumph in the opening game of this NBA Eastern Conference championship series.

He also had three rebounds and a crucial blocked shot in the extra period to finish with some of his most sparkling statistics of the year: 28 points, 18 rebounds, six blocks, two assists and four steals.

It was only the fifth time the 76ers, who tied a league record for most home victories during the regular schedule, have lost in the Spectrum this season. And this was by far the most costly defeat, since it took away their home-court advantage in the best-of-seven series.

The Bullets thought they had the game won in regulation, only to a controversial jump-ball call and a 21-footer by Doug Collins at the buzzer force it into overtime.

The victory was especially sweet because the Bullets felt that almost everyone had written off their chances of beating the high-flying 76ers in this round, especially in the opening game when Philadelphia was coming off a week's rest.

Instead, Washington controlled the tempo of the game from the start and then bounced back after Collins' crushing basket, to quiet the crowd of 13,708, almost 5,000 fewer than capacity.

The only sour note on the afternoon for the Bullets was an ankle injury sustained by the center Wes Unseld late in the fourth quarter. Unseld's ankle was placed in a soft cast in the dressing room and X-rays will be taken today in Washington. The preliminary diagnosis by team physician Dr. Stanford Lavine was a sprained ankle but he was not sure if Unself could play in the next game here Wednesday.

"They could have quit after Collins' shot," said Coach Dick Motta. "It was a frustrating, horrible minute in that huddle, waiting for the overtime to get going."

During that huddle, Motta told his players to forget that they had blown a four-point lead in the last nine seconds of the fourth quarter. He told them, "That's history, so play ball now."

They responded by scoring the first six points of overtime, beginning with a 12-footer by Bobber Dandridge, then a rebound basket by Hayes and finally two Dandridge foul shots after Hayes had blocked a Lloyd Free layup attempt.

Philly called time out trailing, 115-109, with 3:06 to go. The rest helped, as the 76ers quickly got back into it with a jumper from Collins and a fast-break layup from Julius Ervin off a Collins pass.

Hayes now took control. He got the ball inside at his familiar low-post position and turned for a shot against Harvey Catchings, who had just entered the game. Catchings fouled him and Hayes, who had missed three free throws in the last 2:20 of the fourth quarter, made both tries.

After a twisting left-handed spin shot by Erving, who had 25 points for the day, Mitch Dupchak failed on an eight-footer. Kevin Grevey tipped the rebound, Hayes grabbed the ball and laid it in as Catchings fouled him.

Again, he connected on the pressure foul shot with 72 seconds remaining for a five-point margin. Collins then missed a badly forced jumper, Kupchak got the rebound and Hayes clinched the triumph with a rebound basket at the other end.

"They made us play like we were on the road," said Erving about the Bullets' performance. "They were in control the whole time."

Washington knew that it had to control things to subdue the deeper 76ers. The Bullets wanted to fast break when they could, set up their offense the rest of the time and make sure they rebounded well enough to keep Philly from running.

"We did all those things almost perfectly," said Motta. "At the half, when we were tied (at 47 all), I told them we could kick their rears if we just executed the rest of the way. When we play well, we can compete with anyone. But we also are our own worst enemy."

The Bullets wound up running off two more fast breaks than Philly (8-6) while adding 26 layups off their set offense. They committed two turnovers after the third quarter and they outrebounded Philly, 64-57, including 25-17 on the offensive boards.

Dandridge, who had 22 points, did about as good a defensive job on Erving as might be possible and Hayes helped control George McGinnis (five for 16, 13 points 15 rebounds).

The 76er starters were 32 of 74 from the field and only the play of reserves Steve Mix (14 points) and Free (21) kept their club in the game.

Washington's offense benefited from the resurgence of Grevey, who was coming off a frustrating series against San Antonio's George Gervin. Grevey scored 26 points, popping in enough outside shots to keep Philly from sagging too much inside on Hayes, Dandridge and Kupchak (15 points, 14 rebounds).

Yet Philly still almost pulled out the game when the Bullets let down in the final seconds of regulation.

Washington seemingly took control after the game was tied at 105. Grevey banked in a layup off a feed from Uuseld, who twisted his ankle on the play competing with Free for a loose ball. Free than was fouled by Charles Johnston with 11 seconds left, but missed both free throws, the second intentionally.

Bullet rookie Greg Ballard, who had replaced Unseld, got the rebound off Free's second miss and was immediately fouled by Henry Bibby. Ballard, who played only two seconds, sank the two foul shots with nine seconds remaining and his teammates began celebrating.

But Darryl Dawkins stuffed in a rebound of an Erving miss and then referee Earl Strom ruled that McGinnis had tied up Tom Henderson on the inbounds pass.

"I had the ball on my hip, and he had to reach around me to tie me up," said Henderson. "He had to foul me to do do it." But Strom said Henderson had the ball tucked against his chest and McGinnis legally tied it up.

McGinnis won the jump, tipping the ball to Collins, who tossed up an off-balance jumper that went in as the buzzer sounded to force the overtime. It was the only shot Collins took in the quarter.

"We took their best shots and still came back," said Hayes. "We blew a nine-point lead when their reserves made a run at us and then we survived Collins' shot.

"To do that on somebody's home court, well, I think that says a lot about this basketball team. What else can you ask it to do?"