MIDDLESBROUGH, United Kingdom (AFP) - Son Heung-Min surely erased all doubts about his Tottenham future as his double inspired a 2-1 victory at Middlesbrough in the Premier League on Saturday (Sept 24).

South Korean winger Son was reported to be unsettled after starting only 13 league games for Tottenham last season, but his two goals in their latest success indicated the qualities that prompted them to sign him from Bayer Leverkusen 13 months ago.

Son's goals came in the opening quarter of the game as he built on his hugely impressive performance in last Sunday's game against Sunderland - a display that was described as "incredible" by the former France international Thierry Henry.

Middlesbrough, who slipped to their third successive defeat after an encouraging start on their return to the top light, did hint at a revival when Ben Gibson scored midway through the second-half, but Tottenham held on for a deserved success.

Mauricio Pochettino's team climbed to second in the table and they remain unbeaten in the league this season.

For Middlesbrough, this was a dismal way to mark their 250th Premier League game at the Riverside as Aitor Karanka's team have dropped into the bottom six.

They look equipped to sustain their challenge as they wasted no time stamping their authority on the game.

Only a splendid save from Victor Valdes prevented Dele Alli putting them ahead in the sixth minute as the former Barcelona goalkeeper went to his left and turned over a 30-yard shot from the England international.

Valdes was less impressive 60 seconds later, however, as he demonstrated why even his own manager admits the 34-year-old is no longer as good as in the days when he won three Champions League medals at the Nou Camp.

It is true that the keeper was left exposed by a defence that allowed Son to take a pass from Vincent Janssen inside the area and turn away from two challenges before producing a left-foot shot that Valdes touched but couldn't keep out.

It was a moment that raised a question mark over Valdes, and also over Karanka's decision to play Calum Chambers, the defender on loan from Arsenal, instead of Daniel Ayala, one of the most reliable members of his usual back four.

Chambers was blameless, however, when Son added his second goal in the 23rd minute, but two of his team-mates were certainly culpable as Son wriggled clear after being trapped on the byline.

The South Korean moved away from Antonio Barragan and easily eluded a feeble challenge from Cristhian Stuani before steering a shot inside Valdes's far post, this time with his right foot.

There could have been further Tottenham goals before the interval, notably when Christian Eriksen's 35th minute shot was diverted to Moussa Sissoko, who was deprived of a rare goal by Valdes's point-blank save.

And Valdes, who was involved in an angry exchange with team-mate Adam Clayton after more nervy defending, was relieved when Alli's hurried shot wasted an excellent opportunity after a mistake by Chambers that summed up Middlesbrough's uncertainty at the back.

With Tuesday's Champions League tie at CSKA Moscow looming, Tottenham appeared content to cruise through the remaining 45 minutes until Gibson reduced the lead when he rose above Sissoko and headed Stewart Downing's cross inside the far post.

It transformed the mood inside the Riverside and, with substitute Adama Traore making an instant impact, Middlesbrough briefly sensed a comeback.

But the nearest they came to an equaliser was a header by another substitute, Jordan Rhodes, that was comfortably collected by Hugo Lloris.