London - Goals by Younes Kaboul and Troy Deeney inflicted a highly damaging blow to Arsenal's title hopes as Watford emerged 2-1 winners in their Premier League clash on Tuesday.
Alex Iwobi got one back for the hosts in the second-half but they couldn't eke out at least an equaliser with the loss leaving them nine points adrift of leaders Chelsea, who the Gunners face this weekend.
The visitors - who are eight points above the relegation places - took the lead through a free kick from Frenchman Kaboul.
He stepped up and opted for power, not placement. His low drive was heading towards one corner when it struck Arsenal midfielder Aaron Ramsey on the back and wrong-footed Petr Cech to end up in the other one.
Arsenal found themselves 2-0 down less than three minutes later.
Etienne Capoue - who like Kaboul played for Arsenal's north London rivals Spurs - burst on to the ball and smoothly slalomed his way past flimsy challenges from first, Francis Coquelin and then Skhodran Mustafi.
His shot was blocked by Cech but rebounded into the path of Deeney who steered it home with glee.
Worse was to follow for Arsenal when injury forced Ramsey to be replaced by Alex Oxlade-Chamberlain in the 20th minute.
It was Watford who were creating the better chances.
The inspirational Capoue again drove past defenders before crossing for Sebastian Prodl to force Cech into a tip-over after 34 minutes.
Cech made an even better stop moments later, acrobatically turning away a Daryl Janmaat effort.
Wenger, whose players were booed off at half-time, brought on Theo Walcott upon the restart.
The sense of urgency in Arsenal's play was immediately apparent with Walcott shooting narrowly wide. Two minutes later another Walcott effort following a flowing move was blocked by the legs of Watford keeper Heurelho Gomes.
Mustafi then headed wide from a corner before Gomes denied both Iwobi and Mesut Ozil in quick succession, the first with a brilliant reaction stop low down to his right.
The breakthrough finally came in the 58th minute. Alexis Sanchez had been all but invisible in the first half but he drove to the by-line before looping a delightful pass into the path of Iwobi whose volley was ill-timed but deceived Gomes nonetheless.
Watford were struggling to relieve the pressure. As they became ever more desperate in defence, Prodl was booked for upending Sanchez on the rain-sodden surface.
As the minutes ticked away, however, Arsenal were also guilty of snatching at chances with one Gabriel shot lifted high into the night sky.
Substitute Lucas Perez came closest to snatching an equaliser five minutes from time.
Sprinting on to a throw-in, he struck a rising drive which thudded against the underside of the bar and away to safety.