The 22-year-old was in superb form in winning his U69 kilo round of 16 bout against Adolphe Sylva on a unanimous points decision in the Ukraine.
It followed on from this afternoon's victory for twin Luke in the U64 kilo category against Spain's Johann Eduardo Ojeda.
There was double cause for celebration for the Washington brothers - the last eight boxers in each division qualify for the World Championships later this year, meaning the duo have booked their spots for Germany.
Pat faced a difficult hurdle this evening in the shape of rangy southpaw Sylva, but the Birtley ABC fighter cleared it in style.
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The opening round was a cagey affair with both boxers tending to fall short of the target with their punches.
However, when Pat found the mark, he did it in style, driving a three-punch combination into the head of his rival.
Sylva appeared to be feeling the weight of Pat's shots towards the end of the first and McCormack rocked him midway through the second, forcing the Swede to touch down on the canvas.
Surprisingly, Italian referee, Maria Razzardo, did not rule it a knockdown but it provided encouragement for the GB Boxing man who delivered a constant volley of punches to Sylva's head.
To the credit of the boxer from the blue corner, he regained his composure for the last round but a right-left from McCormack and a stinging follow-up soon put him back on the retreat.
Pat finished strongly and was the winner across the board from the ringside judges. He will now face Russia's Sergei Sobylinskii in Wednesday night's quarter-final.