The 21-year-old South Korean star hit the 18th-minute winner at Dortmund's Westfalenstadion on Saturday as Leverkusen claimed a 1-0 win to stay second in the Bundesliga and four points shy of leaders Bayern Munich.

Son could have easily finished with two goals as he fired wide on a later opportunity in the first-half with the goal at his mercy and the Dortmund defence in disarray.

"It's hard to win against Dortmund and I am really glad I scored," said Son, who has netted nine goals in all competitions for Leverkusen since joining from Hamburg at the end of last season.

"We gave it all we had to win today."

This was the third time in succession Son has finished on the winning team against Dortmund, scoring five goals in the process.

He has particularly fond memories of Borussia's Westfalenstadion having scored twice in Hamburg's shock 4-1 win in February after he also hit two goals in his team's 3-2 win over Dortmund in the Hanseatic City in September 2012.

With six goals in his last four league games, including his first Bundesliga hat-trick against his former club Hamburg, Son is in a great run of form.

He has been impressive under new Bayer coach Sami Hyypia, the ex-Liverpool and Finland defender.

"I just want to stay relaxed and enjoy playing football, I came here with a good feeling," said Son, who has made 23 appearances for South Korea.

Having been routed 5-0 by Manchester United a fortnight ago, Bayer Leverkusen must beat Real Sociedad in Spain on Tuesday and hope United beat Shakhtar Donetsk at Old Trafford, to reach the Champions League's last 16.