Kolisi will lead the Springboks out against France in his 100th Test, joining eight other Springbok centurions, while Erasmus will be in charge as head coach of the Boks in his 50th Test.
Alexander congratulated the Bok duo, who have been at the helm during a golden period for SA Rugby, in which time South Africa won the Rugby World Cup twice.
"Today, we celebrate not one, but two remarkable milestones in the proud story of South African rugby," said Alexander.
"Together, Siya and Rassie's journeys have become inseparable from the modern history of our game and our country.
"Siya Kolisi's century of Tests is a testament to resilience, leadership and inspiration. From humble beginnings in Zwide to lifting the Webb Ellis Cup twice as our captain, it is the story of a boy who dared to dream, of a leader who rose to unite a nation and of a champion who carried the hopes of millions on his shoulders – with humility, courage and grace.
"Rassie Erasmus, in his 50th Test as head coach, has been the architect of a new era. His vision, innovation and courage have reshaped the Springbok identity. He took a team at a crossroads and restored belief, discipline and pride.
"Under his guidance, the Springboks reclaimed their place at the summit of world rugby, reminding us that excellence is born of hard work, trust and bold leadership."
Alexander praised the Bok duo for forging a partnership that transcends the rugby field.
"They have united a team and, in doing so, united a nation," he said. "Their story is not just about victories and trophies, it is about healing divisions, inspiring young South Africans and proving that when we stand together, nothing is impossible.
"Siya and Rassie have embodied the spirit of transformation and unity. They have shown us that rugby is more than a sport – it is a mirror of our society, a beacon of hope and a reminder that dreams can be realised against all odds.
"Siya, your 100th cap is a symbol of endurance and inspiration. Rassie, your 50th Test is a symbol of vision and leadership. Both of you have etched your names into the fabric of our rugby history and into the hearts of our people.
"On behalf of the South African Rugby Union and every supporter across our land and beyond, I salute you both. May these milestones remind us of how far we have come and how much further we can go when we walk together."
Siya Kolisi is thrilled to become the ninth Springbok to reach 100 Test caps, and while he thanked everyone who helped him get here, he added that a strong team performance would make the milestone truly memorable.
While Springbok captain Siya Kolisi is reaching a personal achievement of 100 caps on Saturday, the team still comes first, says coach Rassie Erasmus.
Rassie Erasmus says naming the Springbok side to face France was anything but straightforward, with rotation plans, tactical decisions, and some key absences shaping a number of tight selection calls.
Former Australia scrumhalf George Gregan is counting the losses following the unveiling of Rugby's Greatest Rivalry Tour between the Springboks and the All Blacks.
