Fonsi joined Pausini for a powerful performance of "Inolvidable" at the Kaseya Center.
The guitarist/singer said his bandmates covered for him while he "vomited for a bit."
Singer said returning to the U.K. "fills my heart with joy."
The super producer previously opened for his frequent collaborator The Weeknd on the After Hours til Dawn Tour.
Singer said in a video that he'd been battling a bug all week and things "took a turn for the worse" on Friday morning (Feb. 23).
Experience a live performance of the singer's new album This is Me...Now from your couch.
The duo will present their joint album at Accor Arena on Feb. 25.
George Clinton, Kool & the Gang, Cameo, the Chi-Lites and dozens of other R&B and soul standard-bearers will be on hand for August 31 show.
The gigs at the 80,000-capacity venue built for the singer will no conclude with August 30 and 31 dates.
The singer previously said she would be busy preparing for this year's Strength of a Woman Summit.
