The Americans were rarely challenged in the tournament's opening stages and they carried their momentum to a ninth consecutive win.
The 31-year-old Donovan, who took a four-month sabbatical around the start of the year, rallied the team with a second-half goal and three assists.
He set up Clarence Goodson's opening score in the 21st minute before Joe Corona gave the Americans a 2-0 lead just eight minutes later in front of an approving home crowd at M&T Bank Stadium in Baltimore.
El Salvador's Rodolfo Zelaya converted a penalty to trim the lead before halftime but the US broke the match open after the break.
Eddie Johnson and Mikkel Diskerud added goals aided by Donovan, who netted his own score late.
The Americans will now face the winners of the final quarter-final between Honduras and Costa Rica, who are playing later on Sunday in the same stadium.
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