Nonstop quarantine-free services from San Francisco are being reintroduced from January 2022.

A United Airlines Boeing 787 Dreamliner in the newest livery. (Photo: United Airlines)
Nearly two years after suspending its scheduled passenger flights on the route, United Airlines will once again be shuttling travelers directly between San Francisco and Singapore. The American carrier announced today that four-times-weekly westbound services would begin on January 5, 2022. Eastbound services are due to resume on January 9, allowing customers from Singapore to make one-stop connections to more than 70 U.S. cities. The flights will utilize Boeing 787-9 Dreamliners featuring 48 United Polaris business-class seats and 21 United Premium Plus seats (the airline’s premium economy cabin product), as well as 39 Economy Plus seats and 149 United Economy seats.
Flight UA 0001 is scheduled to leave San Francisco at 10:30 p.m. on Mondays, Wednesdays, Fridays, and Saturdays, touching down in Singapore at 7:40 a.m. two days later after 17 hours and 10 minutes in the air. Favorable tailwinds on the eastbound journey will cut the flight time to 14 hours 40 minutes, and UA 0002 will depart Singapore at 10:10 a.m. on Tuesdays, Wednesdays, Fridays, and Sundays, to arrive in San Francisco at 8:50 a.m. the same morning (thanks to the 16-hour time difference). With a distance of 13,571 kilometers (over 8,500 miles) each way, the San Francisco–Singapore flight became one of United’s longest when it launched in June 2016.
A Vaccinated Travel Lane has permitted fully inoculated visitors from the United States to enter Singapore without quarantine since October 19, and all four weekly flights from San Francisco will operate in compliance with the scheme; this means that only fully inoculated customers will be allowed to board United flights to Singapore.
While the U.S. mandates a negative Covid-19 test result obtained within 72 hours prior to the flight, testing requirements for Singapore are a bit different. Incoming travelers will need to take a professionally administered ART or PCR test within two days of departure, followed by a pre-paid on-arrival test at Changi Airport. Singapore-bound flyers must also have received their second dose of a coronavirus jab at least 14 days before arrival, and provide a digitally verifiable vaccination certificate in English issued by either Singapore or the U.S.
More information here.