Chelsea's 33-match Women's Super League unbeaten run came to an end at the hands of Brighton & Hove Albion, who became the first team to beat the champions in nearly two years.
Megan Connolly's late winner gave Brighton a 2-1 victory, although the Blues remain top of the table after Manchester United also slipped up.
Reading were the surprise 2-0 winners against United thanks to goals from Natasha Harding and Danielle Carter.
Those results allowed Manchester City to close the gap on the leaders, and further dent Arsenal's title chances, with a 2-1 win against the Gunners.
Chloe Kelly was a constant danger, setting up both of City's goals for Ellen White and Lauren Hemp either side of a strike from Caitlin Foord.
Everton and Birmingham shared a 1-1 draw, whilst the bottom two - West Ham and Bristol City - also shared the points.
On Saturday, Aston Villa were 1-0 winners against Spurs, Mana Iwabuchi with the only goal in Villa's first home victory of the season as they moved up to 10th in the table.