Federer in Davis Cup u-turn
Roger Federer of Switzerland reacts at a press conference after losing his men's singles semi-final match against Rafael Nadal of Spain on day 12 of the 2014 Australian Open tennis tournament in Melbourne on January 24, 2014 - by William West
The former world number one, who has slipped two places to eighth in the latest ATP rankings, was omitted from Switzerland's original team sheet.
But his presence on the hard indoor courts of Novi Sad from Friday was confirmed by competition organisers.
Later Wednesday, Wawrinka, the new world number three after his heroics in Melbourne, posted a photo of himself and the 17-time Grand Slam winner with the caption: "Look who I've found in Novi Sad".
The Davis Cup is the only major trophy missing from Federer's sideboard.
This is a rare first round appearance for Federer, he has often played in two opening round ties in the past decade.
The last time was in 2012 when Switzerland were brushed aside 5-0 by the United States on home clay.
Serbia, who lost in last year's final to the Czech Republic, are themselves without world number two Novak Djokovic.