Arsenal suffered a major blow in the Premier League title race as struggling Swansea came from behind to win at the Emirates.
The Gunners had a chance to close the gap on leaders Leicester but lost at home to the Swans for the third time in four years to leave them men six points off top spot in third place.
Joel Campbell opened the scoring for the hosts in a game they dominated, also hitting the woodwork three times, but Swansea who were left smiling as Wayne Routledge and Ashley Williams scored the goals to seal a 2-1 win and move them six points clear of the bottom three.
The visitors went into the game with the disruption of not having head coach Francesco Guidolin on the bench after he was taken to hospital with a chest infection.
And his side, managed on the night by Alan Curtis, survived a near-onslaught of their goal as Arsene Wenger's Arsenal paid for their profligacy.
Olivier Giroud, now without a goal in 10 matches, was one of the main culprits but it was Sanchez who thrashed against the post with the game's first clear chance.
Campbell's cross was flicked on to Sanchez by Mesut Ozil and, after a fresh-air shot, the Chile forward hit the inside of the post with the ball landing in the hands of former Arsenal goalkeeper Lukasz Fabianski.
Costa Rica international Campbell was back in the side for his first league start in a month and it was he who opened the scoring, sliding in to covert Alexis' pass.
Hector Bellerin then stung the palms of Fabianski, who then grabbed the loose ball just in front of the onrushing Aaron Ramsey.
Arsenal, as they have done so often, succumbed to the sucker punch as former Tottenham winger Routledge ran in behind to collect Jack Cork's pass and slide a shot past Petr Cech to level with Swansea's first attempt on target.
Wenger, on the end of ever-growing discontent from home fans, was left fuming as he felt Jordi Amat had fouled Ozil in the build-up to the leveller, and he was to remain frustrated as Giroud was the next to hit the woodwork, hammering a volley against the bar from Per Mertesacker's header down.
The second half began with the same pattern, Sanchez again coming close. Swansea continued to look to break on the counter as Arsenal's own attacks ended with little end product.
Sanchez's touch still eluded him as he failed to connect with a dangerous set-piece when eight yards out, with Wenger turning to Danny Welbeck from the bench in the search for a second goal.
But it was another former Tottenham man who almost had Swansea ahead as half-time substitute Gylfi Sigurdsson rounded Cech but sent a shot wide from a narrow angle.
Sanchez hit the frame of the goal for a second time as his free-kick beat Fabianski but shook the crossbar.
Then the body blow to Arsenal's title ambitions was delivered in scrappy style by defender Williams.
Gabriel fouled Andre Ayew and Sigurdsson's free-kick was missed by Cech, with the ball bundled home by Swansea skipper Williams as he left Arsenal to rue another poor display.