Luke Campbell rated his one-point win over old foe Detelin Dalakliev as the most important of his career as he became the first Great Britain boxer to guarantee a medal at the London Olympics with a 16-15 quarter-final win.
Campbell, who had beaten the same opponent to win his European title in 2008 and again in last year's World Championships, said his nerve-racking win in front of a home crowd meant more to him than any other.
Campbell said: "When I've beaten him in the past that's been and gone but tonight comes round just once every four years. I'm overwhelmed at the moment and I'm proud of myself that I'm going to bring a medal home to my little boy."
Campbell, who will now fight Japan's Satoshi Shimizu on Friday with a place in the Olympic final at stake, produced an improved performance from his first-round win over Jahyn Parrinello but acknowledged there is still room for improvement.
Campbell had looked busier in the opening round, staying at range and spearing the shorter Dalakliev, a 2009 world champion, with a pair of long right hands, but the Bulgarian covered up well and shaded the first 4-3 on the judges' cards.
The Hull man continued to rely on his rangy rights at the start of the second, but Dalakliev fired back with renewed accuracy, bulling inside and unloading scoring uppercuts, as Campbell found it hard to avoid walking onto his hard, stabbing shots.
Heading into the final round one point behind, Campbell upped the pace but the wily Bulgarian continued to appear to match him, Campbell's wide hooks bringing roars from the crowd only for Dalakliev to reply with a hard left.
In a frantic final 40 seconds, both men scored with wild blows, Dalakliev clattering home a right and Campbell landing a left almost simultaneously before both men raised their arms hoping for victory at the final bell.
Campbell added: "All the sacrifices and discipline I've had to put in is all worthwhile, but this is just the start. I'm here for gold. I train to win and I fight to win, and I'm going in to win my next fight and then to win my gold medal."
Campbell could be joined by as many as eight fellow Great Britain boxing medallists after Liverpool's Natasha Jonas became the last home fighter to book her quarter-final place with a 21-13 win over American Queen Underwood.