Former Manchester United and Watford striker Ighalo scored his first goal inside 90 seconds to set Al Hilal on the way to the final against Japan's Urawa Red Diamonds where they will seek a record-extending fifth trophy, AFP reported.
Mali attacker Moussa Merega also scored two as Al Duhail coach, former Argentina star Hernan Crespo, was quickly made to regret his pre-match comments that he had a "strong belief" that his side could go through.
The Duhail defence was torn apart from the start. The 33-year-old Nigerian headed into an empty goal to open the rout and had an equally easy tap-in for his second after 10 minutes.
"For people watching the score must have been a surprise," said Ighalo.
"We pushed from the first minute and got a goal and the goals just kept coming so I am very happy," added the striker who walked out with the match ball.
"We know they are a good team and the plan was to keep pressing them high from the first minute."
Duhail's Brazilian defender Luiz Junior said the Qatari side just "switched off for 20 minues" when Al Hilal scored four goals. "It was impossible to come back, but if we played again it would not be the same score."
Marega, who played for Porto in Europe's Champions League, was given a simple shot for the third goal in the 14th minute.
Marega shot the fourth goal into the corner and cleverly backheeled for Salem Al Dawsari to make the score 5-0 at the break.
Ighalo kissed the ground in thanks as the half-time whistle blew as disconsolate and disgusted Al Duhail fans started leaving.
Crespo changed goalkeepers but Ighalo lobbed over replacement Mohammed Al Burayk two minutes after the restart to claim his hat-trick.
Ighalo shot his fourth goal in the 62nd minute before being substituted by Ramon Diaz, who used to be Crespo's coach at River Plate.
The two-match final will be played on April 29 and May 6. Al Hilal beat Urawa when they last met in the final in 2019.