When Chicago resident Peter Mui and his wife, Lily, stepped into their room at the Four Seasons Hotel here the day after Christmas, they found gifts by their bed.

One was a box filled with heartfelt letters of thanks from the friends, parishioners and bishop of Catholic priest Luke Fong, whose life Mr Mui had saved. "It's overwhelming to be on the receiving end of all this," he said.

He had donated his stem cells to Father Luke, an assistant parish priest at the Church of the Immaculate Heart of Mary here, who was diagnosed with a potentially fatal strain of leukaemia in 2013.

Six months after the diagnosis, the Bone Marrow Donor Programme here, which is linked to an international registry, found a match in Mr Mui, now 47, a hospital pharmacy manager living in the Chicago suburb of Glenview.

"We knew this was the right thing to do," said Mr Mui of how his family supported his decision to go through with the donation. In July, 2013, he went through a stem cell harvest in a procedure similar to blood donation, and the cells were hand-carried to Singapore.

They met Father Luke's family for the first time after arriving here on Dec 26.

"Life is short and it would be a real shame if anything happened to (Father Luke) before we had a chance to come visit," said Mr Mui. He is here with his wife, Ms Lily Cho-Mui, 44, and two children, aged 10 and 13, and will leave Singapore tomorrow.

On Sunday, Mr Mui and his family, who are Christians, joined Father Luke for mass at the Church of the Immaculate Heart of Mary, near Kovan.

The two men first met in April, when Father Luke went to Chicago to thank Mr Mui and his family in person. This was after many conversations online, via e-mail, Facebook and Skype, where they learnt about each other.

Meeting Mr Mui in person was a significant moment for Father Luke, who said: "I had this amazing experience where I could see my earthly saviour face to face."

He gave Mr Mui a watch. It was a gift from his grandfather and Father Luke had these words engraved on it for Mr Mui: "A gift of time for time given". Mr Mui has worn it every day since and they have stayed in touch.

The duo have discovered one coincidence - on June 20, 2001, the day that Father Luke was ordained a priest, Mr Mui had his cheek swabbed for his DNA.

Becoming a donor was Mr Mui's way of honouring his childhood friend, Mr Jon Grebin, who died three months after being diagnosed with leukaemia at age 16. Mr Mui said: "I never really had a proper way to say goodbye."

He compares the chance to donate his stem cells to "winning the lottery". The odds of finding a match are around one in 20,000.

Father Luke intends to cook for Mr Mui's family when he goes to Chicago again next April.

He said: "Every day when I wake up and I pray, I look at my hands. These hands have something different in them: the saving cells of Peter. He's with me all the time."

•To find out more about the Bone Marrow Donor Programme, go to http://bmdp.org/be-a-donor