Stefano Okaka scored twice as Watford came from behind and held on to consign Everton to a sixth successive winless Premier League away match with a 3-2 triumph at Vicarage Road.
Romelu Lukaku put Everton ahead after just 17 minutes with his first top-flight goal since October 30, but Okaka helped secure an impressive turnaround with Walter Mazzarri's team benefiting from their opponents' lacklustre defending.
Belgium international Lukaku tucked home from a pass by Gareth Barry - the midfielder moving joint-second with Frank Lampard in the all-time Premier League appearance list with his 609th outing - but Okaka restored parity before half-time when he brilliantly flicked Nordin Amrabat's cross home with his trailing leg.
The Italian evaded Ashley Williams to take the chance, and Sebastian Prodl made the most of similarly slack marking by Seamus Coleman to head Jose Holebas' free-kick beyond Maarten Stekelenburg before the hour mark.
Another dead-ball delivery from Holebas enabled Watford to extend their advantage five minutes later, Okaka getting away from Barry to complete his first Premier League double.
Everton, without a win on the road since September 12, pulled one back through Lukaku in the closing stages but were unable to fully recover and could end the weekend in the bottom half, while Watford, temporarily at least, rise to seventh.
Gerard Deulofeu did his best to make up for the absence of the injured Yannick Bolasie with some teasing deliveries from the right, but it was a more direct approach that resulted in Everton scoring the opening goal.
Barry's simple ball over the top caught Miguel Britos - picked ahead of Christian Kabasele - flat-footed and Lukaku took full advantage by slotting through the legs of the onrushing Heurelho Gomes.
Troy Deeney was unable to control a cutback from Amrabat as Watford responded positively to going behind and they were rewarded in the 36th minute.
Leighton Baines was caught out of position by Amrabat, whose cross beyond the covering Ramiro Funes Mori was skilfully flicked past Stekelenburg by Okaka.
Deeney spurned a glorious chance before half-time, but Watford went ahead after 59 minutes when Prodl got away from Coleman to power a header from Holebas' free-kick into the back of the net.
Everton crumbled and, after Britos was denied by an excellent Stekelenburg save, fell two behind from another set-piece five minutes later, Barry losing Okaka as he darted to the near post and glanced Holebas' corner inside the opposite upright.
Koeman sent Ross Barkley, Aaron Lennon and Enner Valencia on in a string of attacking changes. It had the desired effect when Lennon collected the ball after Watford failed to adequately clear a corner and picked out Lukaku, who nodded home in the 86th minute to set up a tense finale.
Everton may have felt they deserved a penalty when Britos climbed over Lukaku to head a cross from the left clear, but referee Anthony Taylor was unmoved and the Toffees will go into big matches against Arsenal and Liverpool next week without a win in five.
Key Opta stats:
- Watford registered their first win over Everton in any competition since March 1987 (D2 L9 beforehand).
- Everton have conceded 11 goals in their last four away games in the Premier League, including three or more on two occasions.
- At the age of just 23, Romelu Lukaku has now scored against 22 of the 29 different opponents that he has faced in the Premier League.
- Watford have had 11 different goalscorers in the Premier League this season, a joint-high alongside Liverpool.
- Stefano Okaka netted his first Premier League goal in 2414 days; last scoring in May 2010 for Fulham against West Ham.
- Everton have now lost their last four Premier League away games, marking their worst such run in the competition since January 2015 (five defeats).