While everybody at Belmont Park was watching arch rivals Slew o' Gold and Bates Motel, and their two jockeys intently watched each other, Highland Blade sneaked inside them both and won the $400,000 Marlboro Cup today.

Jockey Jacinto Vasquez scored this 10-to-1 upset by steering Highland Blade through heavy traffic on the turn and rallying through the stretch to beat Slew o' Gold by a neck. But mostly he won by minding his own business and avoiding the gamesmanship that preoccupied his rivals.

"I just worried about my horse," he said. "I don't care about Cordero or McCarron."

Angel Cordero Jr. had been widely praised for winning the Woodward Stakes aboard Slew o' Gold three weeks ago, when he completely outrode Bates Motel's jockey Terry Lipham. When the illustrious Chris McCarron replaced Lipham today, the second leg of New York's "fall championship series" was billed as a confrontation of jockeys as much as horses.

Cordero warmed to the challenge; no rider in America is more competitive. For every step of this 1 1/4-mile classic, he rode a tactical race against Bates Motel. He proceeded to win the battle but lose the war.

Slew o' Gold broke sharply, but Cordero put him under tight restraint as he sat outside the early leader, Hyperborean. He stayed way outside. He wanted to be in a position to obstruct the wide-running Bates Motel when McCarron chose to make his move.

Slew o' Gold took the lead on the turn and drew away from Hyperborean. Under normal circumstances, Cordero--or any jockey--would have angled to the rail to save ground. But Cordero kept his mount at least 20 feet off the rail--losing ground that would later seem very important.

When McCarron asked Bates Motel to move, the big California colt did not accelerate with his usual verve, but he still moved into contention outside the leader, putting him as much as 25 feet off the rail.

Meanwhile, Vasquez was thinking only about Highland Blade. He had been racing next to last most of the way, but as he started to accelerate on the turn he had a wall of horses in front of him. Momentarily, he looked as if he might have to swing outside everybody else, but then, he said, "The horses spread out, and I had room to go inside. We had no traffic problems at all."

As he turned into the stretch, one horse-width removed from the rail, he had clear sailing.

Highland Blade never had been considered in the class of the two horses in front of him. He was regarded as a grass specialist, although he did win the prestigious Brooklyn Handicap on the dirt here this summer. But on any surface he has always been a powerful finisher, and today he caught the two leaders midway through the stretch.

Slew o' Gold fought back with determination, but Highland Blade beat him by a neck, with Bates Motel another half-length behind. Deputed Testamony was fourth.

The winner, who paid $23.80, ran the 1 1/4 miles in 2:01 1/5. On the basis of their performance in the Woodward, both Slew o' Gold and Bates Motel had figured to run faster than that today, but they evidently are tailing off after long hard campaigns. "A jockey can't win without a good fit horse under him," Vasquez said, "and I had one today."

It also helps if the horse has a good, astute jockey who minds his own business.

Earlier on the card, Lucky Lucky Lucky, a 10-to-1 outsider, outdueled 1-to-5 favorite Miss Oceana through the stretch to capture the $136,400 Matron Stakes for 2-year-old fillies.

Lucky Lucky Lucky, with Cordero up, scored by three-quarters of a length over Miss Oceana, who finished another three lengths in front of Buzz My Bell. The winning 2-year-old daughter of Chieftain was timed 1:23 3/5 for the seven furlongs and paid $23.60, $4.60 and $2.10.