27-year old left-back Park, who joined from Swiss champions FC Basel in the summer, opened the scoring in the 24th minute when his 30-yard effort took a deflection and looped in over Freiburg keeper Oliver Baumann.
24-year old midfielder Koo, who arrived at the Coface Arena from Wolfsburg in January, came off the bench to score a wonderful goal in just his second appearance for die Nullfunfer.
Koo, who had replaced Yunus Malli just before the hour-mark, controlled the ball with his chest outside the area before advancing towards Freiburgâ€™s goal and then curling a sublime left-footed effort over Baumann with four minutes remaining.
Fellow Asian star Shinji Okazaki, of Japan, was also in fine touch for Mainz and was close to scoring early on but had his effort disallowed for offside.
The victory saw Mainz, now unbeaten in five league matches, move up to seventh on the Bundesliga table as Freiburg dropped into the bottom three.
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