After a goal from each side in the first four minutes, Dortmund and Mainz were inseparable until stoppage time, when Reyna tapped in from a Sebastien Haller flick-on in the third minute of added time.
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Former West Ham man Haller was featuring in his second BVB game after a return from testicular cancer treatment, and registered his first goal involvement.
Reyna's goal sent the travelling Yellow Wall into rapturous scenes, just three days on from doing the same at home in front of the real thing.
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The USA international had earlier ensured a winning return for Dortmund after the World Cup break, bagging an absolutely wondrous strike to finish the scoring in a 4-3 win over Augsburg.
Edin Terzic's team are now one of five teams within five points of league leaders Bayern Munich, whose return to action has seen them take only two points from two games.
And that's with huge thanks to 20-year-old Reyna's undoubted talent, which surprisingly wasn't on show in Qatar, leading to plenty of drama.
Hours on from Dortmund's win, his national team took on Serbia in a friendly in Los Angeles, but without head coach Gregg Berhalter.
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The USMNT manager is currently suspended pending an investigation, due to a story leaked by Reyna's mother, Danielle.
The wife of former Manchester City midfielder, Claudio Reyna, admitted she was the source of a story about Berhalter allegedly kicking his wife in 1991, which he later admitted to.
She also revealed that her decision came after hearing Berhalter discuss her son Gio's professionalism in an off-the-record management conference.
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Gio Reyna played just 52 minutes at the World Cup in his country's four games, leading to plenty of speculation back home.
He plays a similar role for Dortmund, more often than not coming off of the bench, but according to US soccer great Alexi Lalas, the drama isn't over.
After both of Reyna's winners, he put his fingers in his ears in celebration, to 'block out the noise' something which Lalas thinks has plenty of meaning.
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Speaking on his State of the Union podcast, Lalas said: "Everything that he does takes on added and more significant meaning, especially when you celebrate like this - and don't think for a second that players don't have motives by the way in which they celebrate.
"It is that moment where you know every camera and eyeball is going to be on you and what you do is going to be interpreted and at times misinterpreted.
"To Gio Reyna I say this: Gio, I would love nothing more than to not have to hear another word about the crap that you and your parents pulled during the World Cup.
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"Unfortunately, it is not going away and not going away anytime soon, so we are going to have to continue to hear it. And obviously by your reaction, you are hearing it.
"I actually am happy that you at least recognize that you need to be able to cut that out as to the extent that you can and go on. I'm happy that you are getting playing time. I'm happy that you're scoring a banger like this, that is wonderful. That was never, ever the question.
"Ultimately, Gio Reyna has moved on from the World Cup. We are not moving on, and that he's doing this celebration guarantees this story will continue on - which is what we are talking about right now.
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"So mission accomplished if that's what you wanted to do, Gio."