Image source, Getty Images
Raducanu and Alcaraz were given a wildcard for a shot at the winning prize of $1m (£740,000).
BBC Sport tennis news reporter in New York
Emma Raducanu and Carlos Alcaraz might have lost in the revamped US Open mixed doubles - but they certainly won the hearts of the New York crowd.
In a party-like atmosphere at Arthur Ashe Stadium, Britain's Raducanu and Spain's Alcaraz saw their chances of claiming a $1m winning prize ended by fellow Briton Jack Draper and Jessica Pegula.
Despite the presence of home player Pegula, it was the losing pair who were heavily backed by a sizeable crowd at Flushing Meadows.
Pegula and Draper won 4-2 4-2 in the new shortened format, which is part of the US Open's "reimagination" of the mixed doubles.
As well as the huge financial incentives, the event has also been moved forward to the week before the singles main draws to attract the stars.
Raducanu and Alcaraz teaming up for the glamorous event - which has controversially deviated from its traditional format and polarised opinion - created many of the pre-tournament headlines.
There was a fear they might not turn out at all, given Alcaraz played in the final of an ATP event in Cincinnati on Monday.
But the pair walked out together on the US Open's main show court little over 24 hours later to a euphoric reception of screams and shouts.
An excitable atmosphere continued as the fans - who paid between $25 and $145 for what British doubles player Jamie Murray described as a "glorified exhibition" - danced to tunes played by an on-court DJ and acted up when appearing on the big screen.
Both sets of players smiled and laughed throughout, further illustrating how the stars are using the event to tune up for their greater priority of the singles.
For some, it underlined the view of the doubles specialists sidelined that a Grand Slam tournament had lost value.
Pegula, who has been ranked world number one in the women's doubles, and Draper were also a scratch pairing but had too much quality against a pair who are less experienced in tour-level doubles.
For Draper, it was third time lucky - having seen previous planned partners Zheng Qinwen and Paula Badosa withdraw through injury.
Pegula and Draper will play either Serbia's Olga Danilovic and Novak Djokovic or Russia's Mirra Andreeva and Daniil Medvedev in the last eight.