A lacklustre Arsenal suffered their first Premier League loss of the season against an inspired Middlesbrough.
Tuncay (right) celebrates with boss Southgate after putting Boro 2-0 up
Stewart Downing's penalty put Boro ahead after ex-Arsenal striker Jeremie Aliadiere was fouled by Kolo Toure.
Emmanuel Adebayor nodded a rare Arsenal chance wide before Tuncay Sanli angled a Boro shot inches past the post.
But Tuncay fired high into the net to ease Boro to a win, while Tomas Rosicky ended a late Arsenal move with a precise shot for a consolation goal.
Arsenal were punished for producing their poorest performance of the season as their lead at the top of the table was cut to one point after Manchester United won on Saturday.
Aliadiere had failed to make the grade at Arsenal before being sold to Boro in the summer and will have enjoyed his part in his side's opener.
He outpaced Toure to a through ball and touched it past the centre-back, whose leg came across too late and tripped the striker to concede the spot-kick.
Downing squeezed the penalty past keeper Manuel Almunia to add to the strong sense of purpose Boro were showing.
Arsenal looked unsettled at the back and Emmanuel Eboue raised hands at Emanuel Pogatetz but no action was taken for an offence which has yielded a red card in other games this season.
The visitors appeared to be suffering the toll of two tough away games at Aston Villa and Newcastle, whose best spells came when they hunted and harried the Gunners.
Boro adopted the same strategy as Arsenal struggled for rhythm, fluency and potency with the home side the more threatening.
Arsenal were being forced into mistakes by Boro's work ethic and being denied sights on goal by some resolute defending.
Adebayor shows the distress of a bad day at the office for Arsenal
Adebayor's towering header went wide for a poor Arsenal but that only sparked a burst of attacking play from Boro.
They created two promising openings but Tuncay lacked the clinical final pass and the precise shot as the chances went begging.
Arsenal boss Arsene Wenger struggled to keep a lid on his frustration in the dug-out and he hauled Eduardo da Silva off for fellow striker Niklas Bendtner at half-time.
But the move did not make any impact and, instead, Boro should have extended their lead.
Tuncay was clean though but slid an angled shot agonisingly wide of the far post as Almunia closed him down.
George Boateng fizzed a shot wide for the home side before Tuncay reacted quickest to a loose ball in the box - after Gary O'Neil's shot was parried - and side-footed in Boro's second.
The comfort with which Boro held on for a first win in 11 games will worry Wenger, who will also be hoping Cesc Fabregas, Alexander Hleb, Mathieu Flamini and Robin van Persie return quickly.
Rosicky netted an injury-time goal for the visitors who have picked up four points from the last nine.
Middlesbrough: Turnbull, Young, Wheater, Woodgate (Huth 73), Pogatetz, O'Neil, Boateng, Rochemback, Downing, Sanli (Johnson 90), Aliadiere (Arca 81).
Subs Not Used: Steele, Lee.
Booked: O'Neil, Rochemback.
Goals: Downing 4 pen, Sanli 74.
Arsenal: Almunia, Sagna, Toure, Gallas, Clichy, Eboue (Walcott 61), Diarra (Denilson 60), Silva, Rosicky, Eduardo (Bendtner 46), Adebayor.
Subs Not Used: Lehmann, Senderos.
Booked: Toure, Clichy.
Goals: Rosicky 90.
Ref: Howard Webb (S Yorkshire).