The world's number 62-ranked team scored 15 three-point shots on their way to an 89-69 victory in Manila, keeping their hopes of a second-round place alive in their World Cup debut.
South Sudan became an independent country in 2011 and played their first official international basketball game only six years ago.
But Ivey said their win over world number 27-ranked China was fully deserved, as his players "stuck to the game plan from the opening tip to the closing horn."
"They were resilient, persistent, they had laser-like focus, they followed the game plan and competed at the highest level," he said.
"They played with emotion, they played with fight, they played with fire. We competed all 40 minutes and we came out with the victory."
South Sudan led for most of their opening game at the tournament against Puerto Rico on Saturday before losing 101-96 in overtime.
They also made a strong start against China but they kept their noses in front this time, outscoring their opponents by 10 points in the final quarter.
Carlik Jones, who scored 35 points against Puerto Rico, had 21 against China to lead South Sudan in scoring.
Forward Nuni Omot said the win was "a dream come true."
"To be able to do this in front of our fans, people watching back at home, it's been a very emotional day for everyone, getting our first ever World Cup win," he said.
South Sudan can qualify for the next stage if they beat Serbia in their final group game on Wednesday.
Ivey said his team were "not satisfied" with only one win and vowed to give their all.
"My thing is: no pressure," he said.
"My guys play free, play together, they stay together whether we're up 20 or down 20, we don't flinch.
"We're going to come out there and give it our best shot."
Ivey also paid tribute to Luol Deng, the former NBA player who has masterminded South Sudan's rise in his role as president of the country's basketball federation.
"Luol Deng is the heart and soul of all of this," Ivey said of Deng, who played in the NBA for 15 years before retiring in 2019.
"Without Luol, I wouldn't be sitting here right now.
"He had a great vision, I entrusted in his vision and it all came together."