The South Korean internationalist, on loan at the Stadium of Light from Swansea, skipped past two defenders and coolly finished in the final minute of extra time to complete a 2-1 victory and end Jose Mourinho's hopes of winning the trophy which gave him his first honour during his previous spell at Stamford Bridge. That first trophy, the Chelsea manager often says, was the platform upon which many more were won. Not this time around.
Lee Cattermole's own goal had looked to have sent Chelsea through to the last four until substitute Fabio Borini struck with just two minutes of normal time remaining to send the tie into extra time. The Italian might even have won it in stoppage time had it not been for Gary Cahill's desperate challenge as the hosts mounted a late charge.
But Mourinho will have been fuming after the visitors had been in control, squandering good chances to kill the tie off and seal their progress.
Elsewhere, Edin Dzeko pressed his case for a regular run in the Manchester City side with two goals in a comfortable 3-1 Capital One Cup quarter-final win at Leicester. With Sergio Aguero, the club's top scorer, facing at least a month on the sidelines, Dzeko seized his opportunity with a brace, after Aleksandar Kolarov's free kick opened the scoring.
Even though Manuel Pellegrini rotated his squad, the City side he fielded was still too powerful for Leicester, who did manage a late consolation through through Lloyd Dyer. The only downside for City was a troubling injury to key full-back Pablo Zabaleta, who went off with a hamstring problem.