Match report as Kyle Walker opens the scoring with a cross that wriggled home at the far post; Raheem Sterling and Gabriel Jesus added a quickfire double after half-time; Morgan Whittaker scored his first Swansea goal with a late consolation.
Despite a raft of changes, it was a classic Man City performance as they dominated the cup encounter, registering73.6 per cent possession and 17 shots. But they took the lead in strange circumstances as Kyle Walker's cross snuck home at the back post on the 30-minute mark.
Swansea defended well in the first period and had their own sights of goal - Jay Fulton heading just wide immediately after the goal - but were sucker-punched with a Man City double inside five second-half minutes.
Raheem Sterling (47) was left unmarked as he slotted home before Gabriel Jesus (50) added the third shortly after for his fourth goal in six games and second in the FA Cup this season.
Morgan Whittaker's first Swansea goal was a consolation, complete with a fine finish, but it was not enough to inspire a comeback against the Premier League leaders. Man City - who are now unbeaten in 22 games in all competitions - will be part of the quarter-final draw on Thursday evening.