After four-straight wins for the champions, the 3-0 defeat at Amex Stadium on Saturday felt as if it came out of nowhere.
Pascal Gross and Charalampos Kostoulas scored in the first half with shots from outside the penalty area, before Chema Andres completed the win with a header at the back post from Gross' corner as unbeaten Brighton moved third, just two points off the lead.
"There are certain basics that you need to accomplish in football, especially in the Premier League, that we were very far from," said Arsenal's unhappy boss Mikel Arteta, who described it as a "big lesson."
"The amount of technical errors that we did, and we didn't respect any principle of play that you have to, to get out of man-to-man situations."
But it completed a brilliant week for Fabian Hurzeler's Brighton, who won 5-0 at Coventry last week and came from two down in midweek to win 3-2 at Manchester United in the League Cup.
"We have to have the belief to compete with the best teams in the league," Hurzeler said of an outfit who currently look as good as anyone in the English top-flight.
Manchester City, the other team to win their first four games, can go three points clear by beating Sunderland on Sunday.
Fresh off their Stg 300 million-plus ($A562 million-plus) summer spend, Tottenham remain without a win in five league games this season after losing 3-2 at home to Aston Villa.
Johan Manzambi, a 20-year-old attacking midfielder who starred for Switzerland at the World Cup, made his first start for Villa and scored the opening goal against the run of play in first-half-stoppage time.
He set up another for fellow new signing Nicolas Jackson, before Emi Buendia's long-range strike in the 79th sent Villa on their way to a first league win of the campaign.
Conor Gallagher and Jan Paul van Hecke grabbed late consolations for Spurs — their first league goals this season — but it is now two draws and three defeats in the league for Roberto De Zerbi's team, who also lost in the League Cup this week.
De Zerbi, with pressure building on him already, said he didn't speak to his players after the game "because I didn't want to say anything."
"I was very disappointed, very sad, very frustrated for the performance," De Zerbi said, "because we can't lose this game in this way."
New boys Coventry finally won their first Premier League match after a 25-year absence, edging a 1-0 victory at Nottingham Forest, courtesy of Jay Dasilva's 55th minute goal and a late Forest late equaliser from Igor Jesus being denied controversially by a VAR review that
Newcastle scored twice in the first seven minutes — through Lewis Hall and Joe Willock — to set up a 2-1 victory over previously unbeaten newly promoted Hull, while Thierno Barry's 11th-minute goal was enough for Everton to beat Ipswich 1-0.
20°C