The Euro 2016 champions took the lead on 33 minutes through a penalty from their captain, his 75th worldwide goal, before Manchester City-bound midfielder Bernardo Silva knocked in a second from close-range only to pick up an ankle injury which saw him substituted at the break.
After demolishing New Zealand, Portugal progressed as Group A winners and will face the runners-up in Group B-likely Germany or Chile-next Wednesday in Kazan for a place in the July 2 final.
Portugal finish top of Group A on goal difference ahead of Mexico, which also advanced by beating host Russian Federation 2-1 in Kazan.
El Tri will face the victor of Group B in Sochi on Thursday, with Chile or Germany the most likely opponents.
But Portugal did eventually come to life and Ronaldo forced Marinovic into a straightforward save with a header from a right-wing delivery after 24 minutes. "I think we deserved to win and we're delighted with the result and the performance of the team".More news: Raonic loses to 698th-ranked Kokkinakis at Queen's
The pitch remains an issue, too; Ronaldo said it was "difficult" and "the grass wasn't very good" but Santos expressed a hope that it could be improved in the next week.
A report by Canadian sports lawyer Richard McLaren in December said more than 1,000 Russian athletes competing in summer, winter and paralympic sport were involved in or benefited from an institutional conspiracy to hide positive doping tests.
"We want to win this tournament, against whoever", the 29-year-old said.
It was 2-0 four minutes later when Bernardo Silva slipped a shot into an unguarded goal after neat play on the left flank between Ricardo Queresma and Eliseu.
Ronaldo's goal was his 75th with Portugal, taking him level with Sandor Kocsis in second on the list of worldwide goals for European players, and he said he was happy to contribute.
It is the first time since 2001 that the FIFA Confederations Cup hosts have failed to reach the semis.
Bernardo Silva struck three minutes later, but New Zealand nearly pulled a goal back on the hour when Tom Doyle's ball across the face of goal saw Portugal goalkeeper Rui Patricio block from Wood.
However, as the encounter entered the final quarter, Portugal upped the tempo and Nani should have scored from close range, only to see Marinovic pull off a stunning save to divert the ball for a corner.