Mal Acosta of Hayward, Calif., fought off a slump midway through the final rounds of match competition in the $110,000 Fair Lanes Open at Capital Plaza, Hyattsville, last night and defeated his nearest opponent by 165 pins to win top seeding in this afternoon's bowl-off for the $15,000 first prize.

Tommy Hudson of Akron, Ohio, winner of the 1977 Fair Lanes Open, finished second; Larry Laub of Santa Rosa, Calif., winner of the 1979 Fair Lanes Open, was third; Kent Wagner of Allentown, Pa., fourth and Tom Laskow of Commack, N.Y., fifth.

Under the format for today's finals, which will be televised nationally by ABC (WJLA-TV-7 in Washington) beginning at 3:30 p.m., Laskow and Wagner will bowl one game, the winner will meet Laub, and so on to qualify a finalist for the one-game championship match with Acosta.

Acosta, who had total pinfall of 9,608 in the qualifying rounds and in match competition, is assured of winning at least the $8,000 second-place money.

Leading by 108 pins going into the last eight-game block, Acosta almost blew his advantage, scoring poorly in his second, fourth and fifth games of the night (170, 167 and 181). After the fourth game Laub, who also bowled erratically last night, captured the lead by five pins, but Acosta regained it for good in the next game.

"I was failing fast, no doubt about it," Acosta said. "All of a sudden the ball started to hook. I threw it hard and I threw it soft and that didn't help. Then I shifted my feet and that bailed me out."

Acosta regained his composure in the sixth game and held it, scoring 224, 236 and 215 to close out the night.

Hudson had one of his best nights of the tournament and ended with total pinfall of 9,446. But he had the misfortune to lose games in which he bowled 232 and 236, which cost him in bonus scoring. Under the Professional Bowlers Association rules, the 24 bowlers to survive the cut Thursday afternoon are paired in one-on-one matches with the winner of each match to receive 30 bonus pins in addition to the pins he knocks down.

"It was the luck of the draw," said Hudson, who has been in a down cycle recently. "It's going to be good to be back on national television. I haven't done so good lately but I've been bowling well the last three weeks. I've shortened up my grip on the ball and that seems to help."

Laub, who vied with Acosta for the lead for most of match play, dropped to third place with 9,424. Although he bowled six games of 200 and over, he dropped to 145 on the sixth game and 175 on the fifth. Several times he followed consecutive strikes with consecutive open frames.

"I was lost out there," said Laub, who had remarked earlier that his sense of timing has been erratic. "I didn't think I bowled all that well, but then there have been tournaments where I thought I did bowl well and I didn't make the final five so I guess it evens out."

Wagner's total pinfall was 9,391 and Laskow's 9,390.