The Brazilian dented his former club's hopes as Zenit moved onto five points with two matches remaining, one above the Portuguese champions, who are third.
Porto captain Lucho Gonzalez opened the scoring with a fine header after 23 minutes before burly forward Hulk soon punished some lax defending to level.
Helton saved a Hulk penalty in the second period, awarded for a Nicolas Otamendi handball.
Leaders Atletico Madrid host bottom side Austria Vienna later.
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- Lucho Gonzalez