After a tight opening 45 minutes, the game burst into life after the break as Watford scored two goals before the hour mark. The first was scored by Sarr in relatively simple fashion from a yard out after some great work by Abdoulaye Doucoure.
There was nothing simple about his second, though, as he ran through on goal one-on-one with Alisson before sending a delightful dink over the keeper who many consider to be the best in the world.
His runs in behind caused mayhem all afternoon for the champions-elect, and it was via another burst beyond the backline which led to him setting up the third and final goal for Troy Deeney.
The Spaniard played on the left as a wing-back but could regularly be found as Chelsea's most advanced player with his overlapping runs. For his first goal, he patiently held his position at the back post to rifle a strike into the roof of the net after the ball broke to him following a melee in front of the line.
For his second, which came in the 85th minute, he reacted quicker than everyone to pounce on a save from the goalkeeper to send a rebounded header into the top corner from 12 yards.
He remarkably had the most touches inside the opposition box out of anyone this weekend with 11, and sent 27 passes into the final third.
However, they certainly rode their luck, as they conceded 24 shots on goal with eight of them going on target.
Luckily for them, they had Vicente Guaita to rely on as the Spaniard saved all eight of these shots, including two excellent stops to deny Neal Maupay.
The class oozed from the Portuguese all afternoon, and he eventually grabbed a deserved goal just before the hour mark as he was perfectly positioned to tap home after a fizzed low cross from Matt Doherty.
His work for the third really impressed, though, as he glided down the left wing with a confident dribble before cutting inside to thread a pass down the right channel for Raul Jimenez to score the winner.
He enjoyed eight touches inside the opposition box and, incredibly, misplaced just one single pass the whole game.
Sebastian Haller was excellent as West Ham secured their first win in nine to beat Southampton 3-1.
For the first third of the game, it didn't look like it would be his day again as he sent a bullet header at goal which went right down the throat of the keeper.
However, he would eventually earn his place on the scoresheet five minutes before the break as he showed brilliant determination to beat the keeper to the ball in the air before reacting before the ball hit the ground to smuggle a finish over the line from a tight angle.
Thereafter, he very nearly pulled off what would have been the assist of the season as sent Michail Antonio through on goal with an outrageous Rabona, only for his team-mate to miss the chance.
He won a sensational 21/33 duels and 14/21 aerial duels, both of which were the highest tally of any player in this round of fixtures.
Jarrod Bowen (8.3) started his first Premier League match for West Ham since his January move from Hull, and made the most of his chance by scoring the opening goal in the win over Southampton with a clever run in behind before dinking a lovely finish into the corner.
ROUND TO FORGET
Jack Stephens (4.5) didn't give West Ham much resistance inside his own box, as he lost 5/7 duels and 50% of his tackles, while he lost possession a worrying 19 times. He failed to win a single aerial duel as he was bullied and battered by Haller all day.