The Paris Saint-Germain striker had put Uruguay ahead early on by bundling Luis Suarez's cross home with his face after just seven minutes.
According to Federation Internationale de Football Association, at 31 he is the oldest Uruguayan to score a World Cup brace, 108 days older than Diego Forlan who scored twice against South Africa in 2010.
The only downside for Oscar Tabarez's side was the sight of Cavani limping off late on, raising a question mark over his fitness for next week's last eight showdown with the French in Nizhny Novgorod.
Cavani was forced to leave the game in the 74th minute and limped off the field with the assistance of Cristiano Ronaldo.
Cavani scored twice and Ronaldo none Saturday to give Uruguay a 2-1 victory over Portugal.
Bernardo Silva missed a golden opportunity to level the score in the 70th minute after Muslera bungled a save, leaving the ball loose in the box where it fell to the Man City man.
Suarez shimmied and produced a lovely cross right unto Cavani's shoulder cum head and his effort eluded Rui Patricio for the opening goal.
As play switched to the other end, Fonte brought down Suarez for a unsafe 25-yard free kick and the forward's low shot was well saved by Rui Patricio.
With four goals scored after just two World Cup matches, Ronaldo was one of only a few soccer players seen as capable of winning the 2018 Golden Boot - the prize given to the top goalscorer.
Luis Suarez, Lionel Messi, Cavani, Neymar and so many others are leading lights in Europe, so it is crucial for the game that South America goes deep into this World Cup.
'It's not the time to talk about the future, when it comes to the players and coach, ' he said.
Ronaldo barely had a sniff of a chance in front of goal and admitted after the game Uruguay were deserved winners.
Uruguay will play France in their quarter-final on Friday.
Uruguay's hero of the evening, Cavani had to be substituted due to what looked like to be a muscle pull.
Portugal's flank players created some set-piece sutuations but they couldn't yield the desired equaliser as Uruguay saw off the last anxious moments by doing putting in a heavy defending shift.