Editors' pick: Originally published March 4.
Emirates began the service on Wednesday with an Airbus A380 in order to celebrate the milestone, but switched the next day to the Boeing aircraft.
The Boeing 777 now flies seven of the 10 longest flights in the world, according to a list compiled by flight data provider OAG Aviation. The rest are flown by the Airbus A380.
At 8,446 miles, San Francisco-Singapore will be the longest 787 route in the world as well as the third-longest flight in the world and the longest scheduled flight by any U.S. carrier.
The trouble with long routes is that they require big airplanes to carry the required fuel, but when the routes are first being developed it can be tough to lure enough passengers to fill a big airplane.
In the case of San Francisco-Singapore, United has wanted to fly it ever since it merged with Continental in 2010.
"But we said we had to wait until we had the right airplane." said Brian Znotins, United vice president of network, in a January interview. "The airplane has proved it has the legs to do this. The (787-9) has more capacity and more range than the (787-8) and it can carry more fuel."
For now, the longest 787 route is United's 7,923-mile Los Angeles-Melbourne flight, also flown with a 787-9. It is the 23rd longest flight in the world, OAG said. Qantas flies the same route using an A380.
At one time, Singapore Airlines flew non-stop from Singapore to both Newark and Los Angeles using an Airbus A340. Both flights were discontinued in 2013, partially because the four-engine aircraft was a gas guzzler.
At 9,534 miles, the 18-and-a-half hour Singapore-Newark flight was once the longest in the world. Singapore plans to restart the route in 2018 using the Airbus A350.
Emirates said the non-stop flights cut Dubai-Auckland travel time by three hours each way.
The trip has an estimated block time of 16 hours from Dubai to Auckland. The westbound block time is 17 hours and 15 minutes. The 777-200 LR will carry 266 passengers in three classes.
"We anticipate high demand for the route, providing a further boost to inbound tourist traffic into New Zealand that now exceeds three million a year," said Gary Chapman, president of group services, in a prepared statement. "We also expect the service to be popular with New Zealanders seeking faster connections to Europe and the Middle East."
Of the top 10 longest flights in the world, seven are operated by Middle East airlines. Four are between Middle East destinations and Los Angeles International Airport. Two are operated by U.S. airlines, and one is operated by Qantas.