South Korea and Uruguay finished with the same record (1-1-1) in Group H, but South Korea advanced on goals scored (4-2).

Portugal, which won the group despite the loss to South Korea, got on the board first on Ricardo Horta's goal in the fifth minute. Kim Young-Gwon evened it at 1-1 in the 27th minute. In a tightly contested match, both teams had 13 shots, including six apiece on target.

Tottenham Hotspur forward Son Heung-Min, who produced a brilliant assist for the winner after a quiet tournament so far, said he was crying "tears of happiness".

"We waited really long for this moment and we as players believed altogether we could do this," he said as quoted by AFP.

South Korea advanced from group play for the first time since 2010 and only the third time in its history and likely will play Group G winner Brazil.

Portugal won its group for first time since 1996, when it reached the semifinals, and likely will face the winner of Serbia and Switzerland Group G game in the round of 16.

Son, who is still without a goal at this World Cup, ran himself into the ground on Friday to ensure his team reached the last 16.

The Asian side simply refused to accept that their World Cup dream was over and it was captain Son who epitomized the Koreans' relentless energy with a selfless display.

"There were moments when I wasn't able to do my best and I am thankful for my team-mates that they were able to cover me in the moments I wasn't able to fulfil my duties. I'm very proud of them," he said. 

The 30-year-old Tottenham Hotspur forward, who has scored a record 35 goals for his country and is the darling of their fans, who wear his shirt with such pride, has played the whole tournament wearing a black mask to protect a cheek fracture.

With the match locked at 1-1 and the clock ticking into the first minute of stoppage time, it seemed Son's and South Korea's World Cup was about to end in heartbreak with the win they needed to advance about to elude them.

But as he often does for his club, for whom he shared the Premier League Golden Boot last year with Liverpool's Mohamed Salah, he produced something special when it mattered most.

From a Portugal corner, the ball was cleared and suddenly captain Son found himself in acres of space on the lush turf.

Turning on the afterburners he bore down on goal and then, when it appeared the defense had closed him down, Son had the presence of mind to thread a perfect pass to substitute Hwang Hee-chan who calmly slotted his shot past Diogo Costa.

Japan’s maturing tactics

Japan were labelled naive when they exited the World Cup four years ago against Belgium. Now their canny tactics have upset Germany and Spain and taken them to the last 16. The wins also secured the nation group victory for the first time in World Cup history.  

The Blue Samurai will face Croatia in the first knock-out round on Monday after topping Group E with two stunning comeback wins over the European heavyweights.

Japan looked dead and buried when they fell behind in both games but coach Hajime Moriyasu helped turn the tide with his substitutions and tactical changes.

Captain Maya Yoshida credited Moriyasu's counter-attacking game plan for their success and hit back at perceptions that Japan lack the nous to compete on the world stage.

"You're surprised, right?" he asked reporters after the game.

"We're not surprised. You're surprised."

Japan were eliminated in the last 16 four years ago in Russia after throwing away a two-goal lead to lose 3-2 to Belgium.

Playmaker Keisuke Honda was castigated for lofting a last-minute corner into the box with the game poised at 2-2, after Belgium won the ball and raced up the other end to score.

On Thursday, Moriyasu said his plan was to try to hold Spain goal-less until half-time and then bring on speedy, direct forwards to hit them on the counter-attack.

Spain dropped a fly in the ointment when Alvaro Morata gave them an early lead but Japan refused to panic and reached the interval only a goal behind.

Moriyasu brought on Ritsu Doan and Kaoru Mitoma at half-time and the pair caused havoc in the Spanish defense, combining to put Japan ahead within minutes of the restart.

Yoshida said the attacking options at Moriyasu's disposal mean Japan can never be counted out.

"In the second half, both teams were taking more risks and it makes space in behind," he said.

"Even for good center-backs, if fast players attack huge spaces it's always difficult. That was our plan and we had to defend well because their possession was really good."