Tottenham began the season with three wins and a draw from their opening four league matches.
A win on Sunday would have taken them top of the WSL, although that may have only been for a couple of hours with Manchester City, who won their opening four games, playing Arsenal later on.
But London City, given the huge investment they have made in their squad, look the more likely of these two to mount a title challenge.
They finished sixth in 2025-26 but made major signings in the summer, including Putellas and defender Mapi Leon from Barcelona, along with Diani from Lyon.
This was a fourth win in five WSL games, and its manner sends out a message.
Head coach Eder Maestre felt the result and performance would demonstrate to the rest of the league that his Lionesses can make a push for the title.
"The three points are the same as you can have in every game, but this kind of win can be a game changer in terms of showing everyone that we are pushing hard," said Maestre.
"But, at the same time, we need to be humble as well. The teams that are used to being there and occupying these places [at the top] are still there.
"I'm so happy we're hitting our targets. When you have these kind of players it is about trying to find chemistry and it takes time. It's a process and we're in the process of building something strong."
London City will not be in action for another two weeks because of the international break, and will hope that does not affect their momentum.
For Tottenham, the heavy defeat was a reality check.
Head coach Martin Ho said his team "looked scared with the ball" and added: "Really poor, not acceptable from us as a group. Defensively we didn't win a first ball, a second ball. Duels, we were poor. We have to move on.
"The team collectively needs to own that and we will, and that starts with me, I take responsibility for it.
"We were just not at it, but that can't be the level. Everyone will be disappointed - that was not the level we expect. We conceded six goals and you don't want to be doing that."