He played an ill lad battling against Christopher Whitecross, a P6 pupil representing bad bacteria.
An audience of 30 — including pupils' parents, grandparents and friends — saw Rhys come out on top against a rival dressed in a leather jacket and combat trousers.
P5 teacher Aileen Gilbert said: "It was only supposed to be a play acting fight but it looked really dramatic."
The show, which involved Scottish Opera representatives, featured 101 P5, 6 and 7 pupils and boasted songs including The Temperature is Rising, Internal Affairs, Fever, What's up Doc?, In Sickness and in Health and Fight to the Death.
Other characters to appear during the show were good bacteria pupils wearing American football gear, medics, members of the press, and immune system representatives dressed as police.
Carstairs Primary School pupils P4-P5 pupils also wowed parents and grandparents with their 40-minute showing of Fever.
"The show was a unanimous success," said Carstairs Primary P5/6 teacher Jillian Guthrie.
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