Premier League leaders City were in trouble at 2-1 down in the last 16, first-leg tie, playing with 10 men after Nicolas Otamendi was sent off on 68 minutes, before Sane struck.
The Germany winger, a product of Schalke’s academy who left for City in 2016, came off the bench and smashed home a free-kick to equalise on 85 minutes in Gelsenkirchen.
Raheem Sterling then tapped home City’s superb winner on 90 minute, after a brilliant long-range pass from goalkeeper Ederson, to seal victory with the return leg in Manchester on March 12.
“Yeah, I was a bit,” Sane told reporters when asked if he was sorry to score against Schalke on Wednesday.
“I couldn’t realy celebrate, of course I was pleased for my team, but to score a goal like that against my ‘former love’…”
Having waited 78 minutes to come off the bench, Sane was delighted to finally run out at the Veltins Arena.
“It was very emotional for me to be back in my home town,” said Sane.
“It was great to be able to play in this great atmosphere in front of these fans.
“In the first half I had goosebumps, the way the Schalke fans urged their team on.”
City top the Premier League on goal difference while struggling Schalke are down near the relegation places in the Bundesliga.
Yet the Germans gave Pep Guardiola’s City a scare as Nabil Bentaleb netted two first-half penalties, both awarded after the video assistant referee intervened, to cancel out Sergio Aguero’s early goal.
Sane was full of praise for his former club.
“They played very, very well and the way they defended made it hard for us,” said the 23-year-old.
“I was a little bit sad for Schalke, because the atmosphere was amazing, like it always is.
“At the end we did it, we scored three goals away – that was the most important thing.”
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