Son Heung-min has every reason to be constantly smiling, and Tottenham has many reasons to be thankful.

For a footballer so understated, so seemingly undemanding, the forward has become indispensable for Tottenham.

Borussia Dortmund was reminded of that in the Champions League on Wednesday night, when Son scored the opener to spark a 3-0 victory in the last-16 first leg. It was Son's ninth goal in 11 games against Dortmund, including his time in Germany playing for Hamburg and Bayer Leverkusen.

In his fourth season at Tottenham, the 26-year-old South Korean has emerged as one of English football's most influential players. In the 13 games Son has scored this season, Tottenham has gone on to win.

"Next time when Son scores, I go to the dressing room and shower," Tottenham manager Mauricio Pochettino joked, "and wait for the game to finish."

There was little for Pochettino to laugh about a month ago when top-scorer Harry Kane damaged ligaments in his left ankles. But Son is helping to dispel suggestions Tottenham is overly reliant on Kane, inspiring this commanding lead over Dortmund that was secured by late goals from Jan Vertonghen and Fernando Llorente.

Son's contribution — 11 goals in his last 12 games — is even more remarkable considering he started the season at the Asian Games and took a couple of weeks out in January to go to the Asian Cup with South Korea.

"Son is a player that of course has a lot of quality and is a very talented player," Pochettino said. "But he is always available to learn, improve and listen to what you expect from him. He is capable of adapting the quality in different situations."

He proved that right at the start of the second half after Tottenham had been run ragged in the opening 45 minutes and was on the back foot after an error-strewn performance.

In the dressing room, Pochettino got the video clips out during the break.

"We showed a few actions defensively and positions in the offensive situations," Pochettino said.

Within two minutes of the restart, Tottenham was in front after Dortmund left Son unmarked to volley into the net to make it four goals in as many games since returning from the Asian Cup.

"I just had to touch the ball," Son said, praising Jan Vertonghen, who provided the cross.

Son celebrated with a trademark Tottenham handshake, but went over to Serge Aurier rather than Vertonghen.

"I feel sorry," Son laughed after the game alongside Vertonghen, "because I didn't run to him and I ran to the other side."

Vertonghen got his own chance to celebrate as Tottenham scored two late goals inside three minutes.

Vertonghen raced into the penalty area and arrived at the back post to volley in Aurier's cross in the 83rd minute.

"Technically he's just unbelievable," Son said of Vertonghen, who was playing at left wingback rather than in central defense. "He can play every position. He deserves a goal and an assist."

With Dortmund struggling, Llorente scored the third in the 86th when he got a slight touch with his head to turn in Christian Eriksen's corner.

It was a night to forget for United States international Christian Pulisic, who will be playing in London for Chelsea next season, and Jadon Sancho, the 18-year-old former Manchester City attacker who faded in the second half.

"We lost focus," Sancho said. "Son is a good player but hopefully we can handle him better in the second leg."

It is the third setback in a week for Dortmund, which was knocked out of the German Cup by Werder Bremen and then drew with Hoffenheim in the league.

"There are periods of a season when things aren't top, top, top," Dortmund manager Lucien Favre said. "We need to analyze and work hard to avoid and correct these errors."

But Dortmund has a five-point lead in the Bundesliga over Bayern Munich, while Tottenham is five points off the pace in the Premier League in third. This is first time the London team has made the knockout phase in Europe's elite competition in successive seasons. Unlike Dortmund, Tottenham has never lifted the European Cup.

In the night's other game, Real Madrid scored a later winner to beat Ajax 2-1 in Amsterdam.

With a little help from the video assistant referee, defending champion Real Madrid overcame a spirited Ajax to win the first leg of its Champions League round-of-16 match 2-1 on Wednesday and take a big step toward the quarterfinals.

Substitute Marco Asensio scored a late winner to put Madrid in a strong position going into the second leg on March 5.

Real, winner of the last three Champions Leagues, recovered in the second half after being outplayed by underdog Ajax before the break at a packed Johan Cruyff Arena, when the hosts had a goal disallowed by VAR - the first time the video referee had overturned a Champions League goal.

Asensio, a 73rd-minute substitute, was unmarked at the far post as he tapped in a cross from the right by Dani Cavajal with just three minutes of regulation time to go.

"He's shown his development as a player," Madrid coach Santiago Solari said. "He did everything he had to do from the moment he came into the pitch. Not only the goal, which of course was very important for us."

Ajax had looked on course for a draw when Hakim Ziyech shot under Thibaut Courtois in the 75th minute to cancel out Karim Benzema's opener 15 minutes earlier.

The Amsterdam team went into Wednesday's match in something of a form crisis since the Dutch league's winter break. Their previously solid defense crumbled late last month in a 6-2 defeat to archrival Feyenoord and Erik ten Hag's team slumped to a 1-0 defeat on Saturday to Heracles Almelo.

But Ajax shook off its winter form dip before the break, pressing Madrid high up the pitch, combining well with slick, short passes and looking like the team that qualified second in its group behind Bayern Munich rather than the one that struggled in recent weeks.

"We have to improve," Ten Hag said. "Normally you don't create so many chances against a top team like Madrid. We did, but you have to be more ruthless. There we can learn from Madrid — they created less chances but scored more goals."

Real went into the match brimming with confidence from a run of six wins and one draw in its last seven matches. But it only really hit its stride in the second half, proving more clinical in front of goal.

Ajax, a four-time European champion that last won the title in 1995, looked to have taken the lead only to have VAR rule out Nicolas Tagliafico's 37th minute header because Dusan Tadic was in an offside position and impeding Courtois.

Ten Hag was not convinced by the video images he saw.

"I did not see offside and couldn't see a foul on the keeper," he said.

Solari said he had not seen the images.

Kasper Dolberg came close to levelling again in stoppage time, but Courtois stuck out a hand to block his effort as Madrid captain Sergio Ramos was made to work hard for the win in his 600th match for the club.

In Amsterdam, Benzema struck a powerful shot past Andre Onana on the hour after a strong run from the left by Vinicius Junior to put the 13-time European champion ahead.

Madrid's lead lasted 15 minutes before David Neres crossed from the left and substitute Dolberg let the ball run to Ziyech, who shot low under Courtois.

Ajax, in the last 16 of the Champions League for the first time since 2006, had lost its last six matches against Madrid. But it started confidently and Tadic nearly gave Ajax the lead, his 26th-minute shot hitting the post after beating Courtois.

Real created little in the first half. Vinicius Junior came closest for Madrid when he cut in from the left wing and shot on target only for Onana to palm the ball over the bar.