Manchester City and Manchester United are the class of the Premier League this season, but both are on the verge of exiting the Europa League in the last 16 after losses Thursday.
Xandao scored early in the second half as Sporting Lisbon edged City, 1-0, in the first leg of their series in Portugal, while Iker Muniain scored the game- winner at Old Trafford as Athletic Bilbao edged Manchester United, 3-2.
Sporting notched the lone goal 6 minutes after half, as Xandao sent a backheel past City goalie Joe Hart. City created some chances and had more possession, but need a win at Etihad in the second leg March 15 to survive.
United led its match against Bilbao when Wayne Rooney scored the first of his two goals in the 21st minute, but Fernando Llorente, Oscar and Muniain scored consecutive goals for Bilbao.
Rooney added a penalty 2 minutes into stoppage time, but Bilbao escaped with the win and three huge away goals for the second leg in Spain next week.
La Liga sides Atletico Madrid and Valencia also posted wins in their first-leg games, as Eduardo Salvio had two goals in Atletico's 3-1 win over Besiktas and Roberto Soldado scored twice in Valencia's 4-2 win over PSV Eindhoven.
American Jozy Altidore set up goals for Maarten Martens and Erik Falkenburg as AZ Alkmaar defeated Udinese, 2-0, and Luuk de Jong converted a penalty kick as fellow Dutch side Twente edged 10-man Schalke, 1-0.
Mame Biram Diouf notched the tying goal as Hannover tied Standard Liege, 2-2, and David Fuster scored as Olympiacos downed Metalist Kharkiv, 1-0 on the road in the tournament's other first-leg matches.