Louisville, Ky. ? This was payback.

Long shot Giacomo did what his sire, Holy Bull, failed to do when he won the Kentucky Derby on Saturday in the second biggest upset in the 131-year history of America’s most famous race.

It was sweet revenge.

Holy Bull was the favorite in 1994 when a trainer named Nick Zito beat him with a colt named Go for Gin.

Eleven years later, before the second-largest crowd in Derby history, the son, a 50-1 shot, blew past not one or two but five of Zito’s entries and won the $2.4 million Derby by a half-length over Closing Argument.

“I’ve been telling this colt since the first time I got on him, before he ever ran, that he was going to redeem his father’s name in the Kentucky Derby,” jubilant winning jockey Mike Smith said.

He would know, too. The Hall of Fame rider also was aboard Holy Bull.

Zito’s record-tying entries — he had one quarter of the field — included Yankees boss George Steinbrenner’s Bellamy Road, the 5-2 favorite who finished seventh. That was the best Zito’s horses could do.

“I’m human like everybody else, but it’s going to be a little bit of a setback,” Zito said. “When you start thinking about what happens in life, this ain’t that bad.”

Giacomo, left, with jockey Mike Smith up, wins the 131st Kentucky Derby on Saturday at Churchill Downs in Louisville, Ky., beating Closing Argument.

Life is great for Giacomo, who came into the Derby with just a maiden victory in seven starts.

“It almost brought tears to my eyes,” winning trainer John Shirreffs said.

Giacomo finished fourth in the Santa Anita Derby, but Shirreffs was confident his gray colt would run well in the 1 1/4-mile Derby.

Did he ever.

Named for the rock star Sting’s 9-year-old son, Giacomo still was 11th when the field turned for home, but barreled down the stretch to give Smith, at long last, his first Derby win on his 12th try.

Closing Argument, a 72-1 shot, finished second with Afleet Alex third in the largest Derby field since 20 started in 1984. It also was the richest Derby ever run, up from $1 million last year.

The wild results produced the second-highest win payoff in Derby history. Giacomo returned $102.60 on a $2 win ticket. The Derby record is $184.90 by Donerail in 1913.

Under gorgeous sunny skies, and with the second largest crowd in Derby history — 156,435 — roaring as the field turned for home, Afleet Alex looked as if he might win until Giacomo turned on the afterburners.

The winning time for the race was 2:02.75, well off the record of 1:59 2/5 set by Secretariat in 1973.

Zito has to be wondering if he’ll ever have a better chance at winning his third Derby.

Earlier in the week, he said: “If we don’t get No. 3 now, then I’ll have to have Secretariat himself.”

When it was all over, he didn’t hide his feelings.

“I thought I was in good shape. Obviously, it wasn’t our day, and that’s what makes racing. It was a great experience, but a great disappointment,” he said.

The only other trainer to send out five horses in one Derby was D. Wayne Lukas, who won in 1996 with Grindstone, by a nose.

Shirreffs became the third straight trainer to win the Derby on his first try. John Servis won last year with Smarty Jones, and Barclay Tagg won in 2003 with Funny Cide.

Bellamy Road’s disappointing finish comes three weeks after an astonishing 17 1/2-length win in the Wood Memorial. Steinbrenner watched the race from a private suite and wasn’t available for comment, but Zito said afterward the owner told him: “Just keep trying.”

The result probably won’t lift his spirits, even though his Yankees ended a four-game losing streak with a 5-0 victory Saturday over the Oakland Athletics.

Don’t Get Mad was fourth, followed by Buzzards Bay, Wilko, Bellamy Road, Andromeda’s Hero, Flower Alley, High Fly, Greeley’s Galaxy, Coin Silver, Greater Good, Noble Causeway, Sun King, Spanish Chestnut, Sort It Out, Going Wild, Bandini and High Limit.

Giacomo also returned $45 for a place bet and $19.80 to show. Closing Argument, ridden by Cornelio Velasquez, paid $70 and $24.80. Afleet Alex, with Jeremy Rose aboard, paid $4.60.

“I was screaming and hollering for him. He just had nothing left,” Rose said.

If Giacomo goes on to win the Preakness in two weeks, the stage would be set for a fourth straight Triple Crown try in the Belmont Stakes on June 11. War Emblem in 2002, Funny Cide and Smarty Jones each won the Derby and Preakness, but came up short in the Belmont.

Afleet Alex’s trainer Tom Ritchey has already made up his mind. “On to Pimlico,” he said.