It all started out well. South Korea blanked Guinea 3-0 to open group play last Saturday, and then defeated Argentina 2-1 three days later to quickly clinch a berth in the round of 16. A 1-0 loss to England last Friday appeared to be merely a bump in the road, as South Korea had already secured a ticket to the round of 16 and two of their stars, Lee Seung-ho and Paik Seung-ho, were rested at the start.

Against Portugal, though, South Koreans looked flat, and didn't display the kind of jump and energy that had carried them this far.

It can be partially attributed to Portugal's swarming defense, as they double- and triple-teamed South Korean ball carriers before they could create opportunities deep in the offensive zone. And South Korea spent the majority of the first half scrambling to defend the flanks, as the players looked out of sorts in coach Shin Tae-yong's 4-3-3 formation. Only an 81st-minute goal by Lee Sang-heon saved South Korea from getting shut out.

South Korea have never enjoyed a sustained run of success at the U-20 World Cup. The country's best performance remains the run to the semifinals in Mexico in 1983. Counting the current edition, there have been 17 U-20 tournaments since that Mexico stop, and South Korea didn't qualify for six of them, including the previous one in 2015. South Korea have gone as far as the quarterfinals three times, most recently in 2013.

Coached by Shin Tae-yong, this year's U-20 squad wasn't exactly regarded as world beaters. Shin took over the team only six months prior to the tournament, after the team took an early exit at the Asian U-19 championship. In the lead-up to the tournament, the team faced persistent questions about the defense and the players' conditioning.

South Korea silenced critics with solid performances in friendly matches earlier this year. And once the real competition began, the much-maligned defense held its ground until the loss to Portugal. On offense, South Korea rode the backs of Lee and Paik, two FC Barcelona prospects who netted two goals apiece to tie for the team lead.

In particular, Lee, who plays for Barca's Juvenil A, was a revelation. The 19-year-old dynamo has been a polarizing figure among football fans here. The flashy dribbling skills that have so enamored him with fans have also been the fodder for his detractors, who argued the teenager was all about style with no substance. His over-the-top celebrations of goals also ruffled some feathers.

But even non-fans had to admire Lee's goal against Argentina. After taking control of the ball near the half line, Lee sprinted into the offensive zone and left five defenders in the dust before chipping one over the goalkeeper Franco Petroli.

And he tried to carry the team on his back once again in late moments of Tuesday's loss. Lee was easily South Korea's most dangerous and resilient player in the second half as Portugal tried to protect their lead.

Paik, who plays for Barcelona's B team, also did his part, and the two youngsters together thrilled a fanbase that has grown increasingly frustrated and disenchanted with the underachieving senior national team.

And though he didn't score a goal, forward Cho Young-wook, the team's youngest player at 18, had a strong tournament. Goalkeeper Song Bum-keun was the rock as the last line of defense. Center back Jeong Tae-wook was a force on defense for the most part.

Hindsight being 20/20, perhaps South Korea should have sent their best players out against England and tried to keep on rolling into the knockout phase. And it's a lesson that these young players must take to heart -- you can't afford to take your foot off the pedal in tournaments like this. (Yonhap)