Former Barcelona youth prodigy Takefusa Kubo came off the bench and set up Ritsu Doan's winner as Japan came from behind to defeat South Africa 2-1 on Sunday in their Group D opener at the U-20 World Cup.

Grant Margeman handed South Africa the lead in the seventh minute but Koki Ogawa leveled three minutes after the break, and the 59th-minute introduction of the 15-year-old Kubo had a telling impact.

After setting up Ogawa with a glorious chance with his first play -- saved well by onrushing South Africa keeper Mondli Mpoto outside the box, Kubo played his part for their second in the 72nd minute after receiving the ball from Doan and cutting it back for the Gamba Osaka midfielder to sweep home.

"It was high level as expected, there were points in the game where we were overwhelmed with strong challenges but it's huge to win after the whole team came together," Kubo said.

"Doan's pass was a little long but that gave me time to pick the right cross, and I could also hear him shout in Japanese which South African players wouldn't have understood."

Doan said, "Kubo gave me a good pass, we've been working on our combination during training so it was just as we wanted. We won't get complacent (with the win) and will continue to prepare well."

Making their first appearance in the tournament since 2007, Atsushi Uchiyama's men got off to a disastrous start when Margeman beat the offside trap and hit a shot across the goal, which Takahiro Tomiyasu could only divert into his own net.

Ogawa had a left-footed volley denied by the post in the 19th minute before diverting his header wide from Koji Miyoshi's cross on the half hour, while Luther Singh missed the target with just the keeper to beat at the other end.

Ogawa made amends for his earlier missed opportunities after the break as he flicked Yuto Iwasaki's cross in from the left. The effort lacked pace but had just enough to go over the line, with keeper Mpoto unable to scrape it out in time.

Kubo added sparkle to the Japanese attack which was looking more dangerous after the break, but South Africa had their chances. Singh had a loose ball fall to him within 10 meters after a scramble in the box, but Tomiyasu made a last-ditch tackle to deny them going ahead before Doan's clinical finish settled an exciting encounter.

Japan will now face Uruguay on Wednesday before facing Italy in the last game in the group stage.

"I think we made a decent start as a team, and also me personally," Kubo said. "We'll up the gear for the next game and hopefully we can win the next game to go into the third game relaxed."