Korea finally emerged victorious in what was the fifth quarter-final meeting between the two regional giants in as many Asian Cups -- putting them 3-2 up in their mini-series and breaking Iran hearts just as penalties seemed inevitable.[break]
Yoon, 20, the K-League young player of the year, struck in injury time at the end of the first period of extra-time, weaving his way across the face of the box then hitting a curler past the keeper and into the bottom corner.
The other semi-final will be between Australia and Uzbekistan, also on Tuesday.
"The players were willing to sacrifice themselves for the team and we showed good organisation. But the most important thing the players learned was to enjoy controlling the game," said Korean coach Cho Kwang-Rae.
"Japan have been getting better and better in every aspect in this tournament. But I have never feared Japan, even as a player, and I don´t fear them now."
Iran coach Afshin Ghotbi admitted it was hugely disappointing for the three-times champions, who have not won the Asian Cup since 1976.
"In the first-half we didn´t play like we can, that maybe had to do with the Koreans´ quality and our own tension," said Ghotbi, who used to be South Korea´s assistant coach.
"I could´ve changed seven or eight players in the first-half as they really didn´t play as well as they can.
"But as the game wore on we got better and unfortunately we lost a goal at the worst moment, but it was a fantastic finish."
South Korea enjoyed the better of a tight contest, having their first chance after a minute, but Manchester United star Park Ji-Sung, winning his 99th cap, totally missed his volley as his side made the far more confident start.
Iran were physical, conceding a series of free-kicks as they attempted to nullify the likes of Park and impressive attacking midfielder Koo Ja-Cheol, who scored four times in the group stage.
On 24 minutes Lee Yong-Rae fired just wide, then four minutes later Iran came closest to taking the lead when Korean striker Ji Dong-Won inadvertently flicked a Mohammad Reza Khalatbari free-kick back towards his own goal.
But Jung Sung-Ryong pulled off a fine save, diving left and low to claw the ball out. It would have been harsh on the Koreans, whose quick movement and slick passing had their opponents running in circles at times.
Ghotbi had seen enough and at half-time brought on Khosro Heidari to replace Mohammad Nosrati, one defender for another.
Yet there was no immediate sign it made any difference, as the Koreans started the second period in front of 7,111 at Qatar Sports Club Stadium much as they had ended the first -- in control but making few clear chances.
The lanky Ji rose highest but could only head straight at Iran keeper Mahdi Rahmati on 63 minutes, then Khalatbari caused mild panic in Korea´s box in a rare Iranian raid and moments later Ehsan Haji Safi flashed a drive wide.
Ghotbi threw on striker Masoud Shojai, who plays in Spain for Osasuna, with 15 mins left, as Iran belatedly swung the game in their favour. Cho hauled off the fast-fading Koo for Yoon.
It was a masterstroke.
The match dragged into extra-time with the game square at 0-0 and neither team really threatening to break the deadlock, before highly rated midfielder Yoon finally prised the two sides apart after 105 minutes of football.
Australia beat holders Iraq to make Asian Cup semis
A 117th-minute strike from Harry Kewell powered Australia to a 1-0 quarter-final victory over holders Iraq at the Asian Cup on Saturday and into a showdown with Uzbekistan for a place in the final.
The Galatasaray forward met a pinpoint cross from Matt McKay towards the end of extra time and angled his header past Iraqi goalkeeper Mohammed Kassid to settle a match that that could have gone either way.
It set them up for a semi-final against the Uzbeks on Tuesday, and avenged their 3-1 loss to Iraq in the group stage four years ago, when the war-torn country went on to win their first ever continental crown.
Both sides had chances and the Australians laid siege to the Iraqi goal for most of the second half, but they were unable to get a breakthrough in the regulation 90 minutes, sending the tie into a tense period of extra time.
"It´s definitely a great achievement, even though we went into extra time," said Australia´s German coach Holgier Osieck.
"We showed great character in this game. It was delightful for the coach to watch. Basically, we should have done it in the regular 90 minutes. Iraq played better in extra time."
Iraq coach Wolfgang Sidka, who is also German, said: "Certainly we are disappointed. But the way we played was 100 percent OK.
"We created more chances in extra time and it was a very exciting fight for both teams."
Osieck was boosted by the returns of Dynamo Moscow right-back Luke Wilkshire and Blackpool defender David Carney, who were sidedlined with injuries in the 1-0 win over Bahrain last Tuesday.
But Blackburn Rovers midfielder Brett Emerton missed the match due to suspension.
It was a turgid start, with few chances.
Iraqi captain Younis Mahmood had the first real sniff of goal on 24 minutes, attempting a shot on the break, but it floated onto the top of the net without troubling veteran goalkeeper Mark Schwarzer.
McKay then squandered a one-one-one chance against Iraq custodian Kassid when he dashed into the box and latched onto a deflection off a long ball, but he lifted his shot high over the bar.
Neither side was taking the initiative in a scrappy half, but the Socceroos were marginally better.
In the 42nd minute, McKay brushed aside Samal Saleed to trap the ball on the edge of the area but his one-touch right-footer flew high over the bar.
Kewell´s turnaround left-foot shot in a carbon copy attack one minute later was also popped over the net, with the 32-year-old holding his head in disbelief.
Kewell had another opportunity soon after the interval with his shot from the left swerving into the near side of the net.
The Lions of Mesopotamia, who scored just two goals in the group stage, all but opened their account on 55 minutes when Emad Mohammed took the ball in the box but his right-foot shot zipped past Schwarzer and off the post.
The game was livening up as the clock started ticking and a McKay corner was headed back to Sasa Ognenovski in a goalmouth scramble but the big defender could only send it towards the Iraq keeper.
Soon after, Holman sent a screamer from 25 metres bouncing off an Iraqi defender and Kassid was forced to punch the ball clear in the 81st minute as Australia pressed for the winner.
But it was destined for extra time, with Cahill, clearly not fully recovered from the thigh injury he picked up against Bahrain, taken off by Osieck.
Australia should have been 1-0 up on 98 minutes when Ognenovski sent a spectacular overhead bicycle kick towards the goal but Mile Jedinak got in the way and diverted the ball over the bar with his head.
With the game looking like it was going to penalties, Australia shattered Iraqi dreams as Kewell stepped up to become the Socceroos´ hero.
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