A single goal from Luis Suarez was enough to give Uruguay victory against Saudi Arabia, leaving them on six points along with Russia and condemning Egypt and the Saudis to an early exit from the tournament.
In the earlier game, in Group B, Ronaldo netted his fourth goal of the tournament in Portugal's 1-0 win against Morocco, ending the north Africans' chances of progress.
The five-time world player of the year scored with an early header to add to his hat-trick for the European champions against Spain in their opener.
In the other Group B match, Spain needed a fortunate Diego Costa goal to beat a determined Iran side who had a second-half effort from Saeid Ezatolahi disallowed for offside in Kazan.
Portugal and Spain both have four points and will be competing for top spot when they complete their first-round games against Iran and Morocco respectively on Monday.
The top two teams in the group will face Russia and Uruguay in the last 16.
Iran have still never beaten a European side at a World Cup, but coach Carlos Queiroz said they could take pride from their performance against Spain.
"We left the pitch 1-0 down, but we won respect. I'm sure the fans are proud of our team," said Queiroz.
"Our players can go home and sit with their family and friends and be proud of themselves."
Having won their first match, Iran could still secure an unlikely qualification if they beat Queiroz's native Portugal.
Uruguay came up against an improved Saudi side in Rostov-on-Don, but master poacher Suarez took advantage of a goalkeeping error by Mohammed Al-Owais to put his side ahead midway through the first half and the Saudis could not respond.
It was a fitting way for the Barcelona forward to mark his 100th international appearance and he will be hoping to go further in the World Cup after exiting the last two in disgrace after being sent off for a goal-line handball against Ghana in 2010, and infamously biting Italy defender Giorgio Chiellini in 2014.
Morocco, like so many sides before them, were undone by a piece of Ronaldo brilliance. He now has 85 international goals -- a record for a European player.
"I'm happy I scored the goal but the most important thing is that we won the game. We have to think match by match in order to progress," Ronaldo said.
Disappointed Morocco coach Herve Renard said his side had showed they had a bright future despite being out of the running to qualify for the last 16.
"We qualified for a World Cup after 20 years and we've showed that we can play football," the Frenchman said. "We do play football."
Meanwhile, host nation Russia were celebrating the 3-1 win over Egypt on Tuesday which paved the way for them to move into the next round despite pre-tournament expectations that they would struggle to get out of their group.
After a goalless first half, Egypt captain Ahmed Fathi scored an own goal before Denis Cheryshev struck and Artem Dzyuba added a third. Mohamed Salah scored a consolation penalty late on but it was not enough for the Pharaohs.
"Can you believe it?" Moscow's Sport Express newspaper asked.
"We waited a very long time for this," said Sports Minister Pavel Kolobkov.