Sizemore beat former McGlashan school team-mate Paddy Ou 6-4, 6-2 in a match where he never let Ou get close enough to threaten his greater ground stroke power and bigger serve.
In a recent club game between the pair, Sizemore had lost in three close sets as he became tentative at vital stages.
"I was determined once I led 5-2 that I wasn't going to let him back into it this time,'' he said.
"So good,'' he added in relief.
Fields had a similar experience, having lost to Eliza Booth in the recent schools' qualifying event.
"I was pretty motivated to turn that around,'' she said.
She was more solid, winning 6-3, 6-2 in a small field of six.
Both the men's semifinals followed a similar pattern.
Sizemore mixed patience with aggression to gradually overcome Ryan Eggers 6-4, 6-1.
Eggers generally kept a good length, but when he strayed, Sizemore took toll with powerful finishing.
Ou was a little steadier in his first set win 7-5 against Peter Hartono (15), but the younger player gradually fell into errors as the pressure told and Ou won the second 6-1.
There was a hint of an upset that was to come when Aaron Hicks, the top seed, dropped the first set 6-7 to Hamish Low in the first round before recovering to win the next two 6-2, 6-3.
However, Queenstown schoolboy Manawa Rakete-Shea used his fluent ground strokes to power to a 7-5, 6-3 win and eliminate Hicks in the second round.
Rakete-Shea was not up to a repeat when he lost 7-6, 6-4 to Eggers in the quarterfinal.
Ou, Sizemore and Hartono, seeded second, third and fourth ensured there were no further upsets as they accounted for Carlos Reid, Connor van Dalsum and Philip Hooper, the sixth, seventh and eighth seeds respectively.
The men's doubles was won by Sizemore and Reid in an entertaining and tight match when they beat Ou and Eggers 7-5, 6-4 in the final.
The mixed doubles final went to Fraser Richardson and Heike Cebulla who beat Mitchell and Kimberly Heads 6-2, 6-4.
