That uncluttered mind helped him deal with the growing pressure the deeper he went into the event. Matheson was said to be "okay" with his former charge's big upset win.
"I think he was pretty nervous playing me. I was relaxed. He beat the first seed, so he was favoured to win. I didn't feel much until the next day."
But this amazing victory does not mean Guo has shot to the top of the national rankings. He has not played the requisite four tournaments, so his national senior ranking is down at 24 - but he is the top under-17 badminton junior in the country.
It is wrong, though, to say Guo has come from relative obscurity to annex the national title. This year he won the Oceania under-19 singles crown and defended his Australasian under-17 singles title, while also claiming the mixed doubles.
This weekend he will compete in the North Island under-17s at his home court of the North Harbour Badminton Association. Pending selection, he hopes to be at the world juniors championships in Peru come November.
Westlake has a proud and successful recent badminton history - they are the national schools doubles champions and the Auckland champions - and Guo is into his second year in the premiers. He has been coached by Westlaker Chance Cheng, a three-time Westlake badminton champion, and has played with last year's captain Niccolo Tagle. One of his former mentors is Dan Shirley, who won a world championship mixed doubles bronze medal - the only New Zealander to do that well on the elite global stage.
Guo does not play against his parents anymore - he might just be too good for them - but he often has practice matches against much older players, so is used to the idea of competing with more seasoned customers, who sometimes try and get in his head to put him off his game. So far that has not worked too well.
He comes under the watchful eye of Fanny Megahwati at North Harbour and his personal coach Li Feng, a former China and New Zealand rep.
Guo brings an attacking mindset and style to his badminton, which so far seems to be paying off.