If Pat Day had made even one tiny mistake, Chief's Crown would have been draped with black-eyed Susans today. But because America's champion jockey delivered a performance that was close to perfection, Tank's Prospect scored a dramatic photo-finish victory in the 110th Preakness.

Day, aboard Tank's Prospect, saved ground on both turns, made a perfectly timed move when his main rivals were a bit overanxious, and wore down the even-money favorite Chief's Crown in the final yards. He won by a head and matched Secretariat's unofficial Preakness and Pimlico Race Course track record with an official 1:53 2/5 for the 1 3/16 miles. Eternal Prince finished third, 2 1/2 lengths back, after setting a swift early pace.

Tank's Prospect had come into the Kentucky Derby two weeks ago with strong credentials, but had no chance to make his usual stretch run when Spend a Buck controlled the pace and led all the way at Churchill Downs. Trainer Wayne Lukas enlisted Day, the Eclipse Award winning jockey of 1984, to ride him in the Preakness, where he hoped a fast early pace might help his horse's chances.

He got his wish. Practically all 81,235 people at Pimlico knew that Eternal Prince was going to try to lead this race all the way, but he didn't lead it the way jockey Chris McCarron would have liked. Instead of slowing down the pace, Eternal Prince had to go into high gear to get to the front, covering the first quarter-mile in 22 2/5 seconds and the half in :45 1/5.

Those fractions were only one-fifth of a second slower than the fastest in Preakness history, in the 1976 race that is often remembered as a classic illustration of a suicidal pace.

But Eternal Prince's rivals were so afraid of him, and so conscious of the importance of speed in the Preakness, that they were on his heels. Jockey Don MacBeth had Chief's Crown in fifth place, three lengths behind the leader, at the first turn. Day, however, was in no hurry. He angled Tank's Prospect to the rail on the turn and stayed inside near the rear of the pack.

Eternal Prince was still setting a torrid and probably destructive pace as he reached the three-quarter mark in 1:09 2/5, but MacBeth didn't want to wait any longer. He moved outside to challenge the leader, and the two of them battled head-and-head around the turn.

Tank's Prospect was close to Chief's Crown when MacBeth made his move, but Day remained patient and let him go. He steered Tank's Prospect around two tiring horses, then dropped immediately back to the rail and saved ground turning into the stretch. He, like the others, was able to avoid trouble when Hajji's Treasure broke down on the backstretch with a fractured right front leg.

By then Chief's Crown had raced Eternal Prince into defeat and opened a clear lead, passing the one-mile mark in 1:34 1/5, the fastest such fraction in Preakness history. He seemed for a moment to have command of the race, and even Day said afterward, "I wasn't sure if I was going to be able to run him down."

But Chief's Crown was starting to weaken after such a grueling pace, and Day angled Tank's Prospect outside him for the desperate run to the wire. "With every jump he was closing a tremendous amount of ground," Day said. Two jumps from the wire, he caught the leader and won in the closest Preakness finish since 1969.

Tank's Prospect's time for the 1 3/16 miles will be recorded as the official Pimlico track record, breaking by one-fifth of a second the mark set by Gate Dancer last year. When Secretariat ran 1:53 2/5 in 1973, he was deprived of the record by a malfunctioning electric timer.

Tank's Prospect earned $423,200 for owner Eugene Klein, a former owner of the NFL San Diego Chargers. Tank's Prospect paid his parimutuel backers $11.40, $3.40 and $3. Chief's Crown paid $2.60 and $2.40. Eternal Prince paid $3.20 to show. The exacta was worth $24.40.

Tank's Prospect, a son of Mr. Prospector named after former football star Tank Younger, was one of the country's top 2-year-olds last season, finishing second to Chief's Crown in the Breeders' Cup. He was written off as a candidate for the 3-year-old classics after he placed last in the Santa Anita Derby, but Lukas found that he had been suffering from a throat problem that was impairing his breathing. Lukas sent the colt into surgery and then boldly sent him a few days later to Oaklawn Park for the Arkansas Derby.

He won there by 6 1/2 lengths with a powerful stretch run, in one of the top performances by a 3-year-old this season. But he was forgotten again after his poor performance in the Kentucky Derby; all the prerace speculation saw this Preakness as essentially a two-horse confrontation that would be won by either Chief's Crown or Eternal Prince.

That assessment might have proved correct if MacBeth hadn't been in such a hurry to take the lead, or if Day had not been flawless.