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Boro were tumbled off the top and overhauled by rivals Watford after a dismal 2-0 defeat at Vicarage Road.
In a poor day at the office Boro looked creaky and unconvincing and never really did enough to hurt Watford who nudged a point above them at the summit.
Boro started slowly but had some promising spells in the first half but failed to make them count then were punished for some sluggish defending as Troy Deeney slammed home in the first half.
That was the fourth game in a row at Vicargae Road that the striker had netted against Boro and it swung the game decisively their way.
After the break Boro stepped up the tempo but still looked ragged then were dented badly as the conceded a sloppy second, squandering possession cheaply before Odion Ighalo slammed home a rocket.
Boro had some late chances to salvage something but lacked self-belief and punch up front.
Aitor Karanka made a big call and gave debuts to loan signings in defence. Dwight Tiendalli from Swansea started at right-back and Fernando Amorebieta of Fulham was in the middle in place of Tomas Kalas and Ben Gibson.
And up front Lee Tomlin returned in place of Jelle Vossen to play behind Patrick Bamford.
Boro kicked off in bright sunlight and pressed early on as a long hoof upfield from Dimi Konstantopoulos was nodded on by Patrick Bamford for Adam Reach to chase down the left and then Boro passed and probed before a Tomlin ball into the box was hooked away by Guedioura.
Then they won a corner on four minutes after a good work down the right by Albert Adomah but it was scrambled away.
A second corner a minute later was played back out but when Amorebieta chipped it over the defence for Adam Clayton to burst onto, the offsie flag went up.
Watford made their first probe on six minutes as Ighalo chased a quick ball down the left but Jonathan Woodgate was quickly across to collect and clear.
Then after Woodgate nodded away a long ball towards Deeney it landed at the feet of Ighalo who pushed it forward for his striker partner to chase but the alert Boro skipper cut it out.
And on 10 minutes a long ball from Deeney down the inside left channel sent Anya surging into the box after it but Konstantopoulos was quickly out to slide in and gather at his feet.
Boro threatened on 12 minutes as they won a free-kick 30 yards out and Grant Leadbitter's stinging drive sent keeper Gomes full length to save at the near post.
Another free-kick from a similar position on 17 minutes found Bamford but his harmless header flew well wide.
Watford threatened soon after as Anya burst to the edge of the box onto a Watson touch on but Woodgate stretched to toe poke away from him back to the keeper.
And Watford carved out the first real chance on 22 minutes as lively Anya played a one-two with Ighalo to cut into the box from the left but his low 12 yard angled drive was blocked by the legs of the advancing Konstantopoulos.
Then Watford won a corner and when Watson curled to the near post Angella headed just wide.
On 26 minutes Reach was booked for a foul on Tozser.
Watford threatened again on 28 minutes as a high ball forward brought a wayward header that fell for Deeney who played a one-two with Ighalo to get to the edge of the box but his hurried shot flew well wide.
Boro almost broke through on 31 minutes as a Watford player tried to take a quick-free kick on the left which came back of Reach who spotted the run of Bamford down the right and crossed in for the striker to drill in a low angled shot which was turned behind.
Then a minute later Friend swung in a cross from the left and Tomlin put in a 15 yard volley that was well held by Gomes.
And as the pressure continued Adomah wriggled down the right then square to the edge of the box but Tomlin's shot was charged down.
But Watford broke out to draw first blood on 37 minutes.
The defence stood off and left Guedioura time to scoop a cross from the right into the box and when Woodgate misjudged it the ball fell for TROY DEENEY to carve in from the left and past static Tiendalli then slot an angled shot past Konstantopoulos and into the far bottom corner.
HALF-TIME: WATFORD 1 BORO 0
Boro had a good spell on the restart and for a few minute they battled down the left and several times threatened to break through.
And they craved out a good chance on 49 minutes as a deflection and a quick ball by Leadbitter down the flank sent Friend surging to the byline to whip in a low cross to the near post where the diving Adomah could only bundle it wide off his chest.
Watford responded as a quick ball by Guedioura through the middle sent Ighalo clear on the edge of the box to rifle a shot straight at Konstantopoulos.
Boro then probed down the right but a mistimed ball by Tiendalli and a mistimed run by Adomah saw the offside flag go up.
On 59 minutes Watford put on Connelly for Cathcart.
On 61 minutes Boro put on Kike and Tomas Kalas for Tomlin and Tiendalli.
Watford went close on 62 minutes as a high ball into the box was only half headed clear and the loose ball was pushed by Toszer to the edge of the box for Watson to crack in a low shot that deflected just wide of the far post.
And Watford stunned Boro with a second goal on 65 minutes.
Leadbitter dithered on the ball and was mugged out on the left touchline and a poor back-heel sent Watford's Watson free and he squared to the edge of the box where ODION IGHALO spun past static Woodgate and rifled a rocket shot into the far top corner.
On 68 minutes Watford put on Vydra for Ighalo.
On 69 minutes Guedioura was booked for a foul on Friend.
Boro lamost broke through on 74 minutes as Friend surged down the left and scythed in towards the box then played a one-two with Kike but Hoban arrived with a crucial blocking tackle as he went to smash the return.
Watford were immediately back on the attack as Watson carried forward then slotted outside to Vydra over-lapping on the left but he took it too wide and ran it out of play.
Good work by Bamford on the right forced an opening on 77 minutes as he won a throw then quickly picked out Kike who held it up then laid it back to Reach but his wayward shot flew into a crowd and deflected away harmlessly.
Then Kike headed over from a Kalas cross as Boro found a bit of urgency.
But Watford went close to a third on 81 minutes as Deeney fired just over on the turn then a minute later as Boro failed to clear their line a ball dropped for Guedioura 20 yards out and his volley sent Konstantopoulos stretching to tip away from the top corner.
Straight up the other end Friend again cut into the box to fire wide of the far post under pressure.
Boro made their final change on 84 minutes, Adam Forshaw replacing Clayton.
Boro had a shout for a penalty in the final minute as Bamford tussled with Connelly in the box and the Watford man appeared to handle but the ref waved appeals away.
And seconds late Boro went close as Friend played the ball into the box for Kike to smack in a first time low effort that brought a good full length save from Gomes.
In stoppage time Watford put on Layun for Guedioura.
Then to add to a poor day Leadbitter was booked for pulling back Layun off the ball after the Watford man had fouled him, picking up a needless two match ban. Layun was also booked.
BORO (4231): Konstantopoulos, Tiendalli (Kalas 61), Amorebieta, Woodgate (c ), Friend, Leadbitter, Clayton (Forshaw 84), Adomah, Reach, Tomlin (Kike, 61), Bamford. Subs: Ripley, Gibson, Whitehead, Vossen.
WATFORD (352): Gomes; Cathcart (Connelly 59), Angella, Hoban; Paredes, Tozser, Watson, Guedioura (Layin 90+1), Anya; Deeney (c ), Ighalo (Vydra 68) Subs: Bond (GK), Munari, Pudil, Abdi.
Ref: Mike Jones (Chester)
Att: 19,656 (including 1,916 Boro)
