What happens when an unstoppable force meets another unstoppable force?

Both teams have been in irresistible domestic form - Chelsea have won nine straight games in the competition, and are the WSL's top scorers with 31 goals from 10 games.

Arsenal are three points behind in second place, having played one game fewer, and likewise have lost only one league game this season - at home to Manchester United in November.

It is in European competition in which both have really caught the eye, having reached the Champions League quarter-finals by winning tough groups.

Arsenal progressed ahead of reigning champions Lyon, having stunned the eight-time winners 5-1 in France in their opening group game.

Chelsea, meanwhile, strode through Group A with 16 points from six games, making light work of a pool also containing PSG and Real Madrid to exorcise the memory of their group-stage exit 12 months ago.

Former England international Anita Asante, who played for both Arsenal and Chelsea, thinks there is some merit to the argument that their meeting is the big one in European women's club football.

She told BBC Sport: "Whether either side is the best in Europe, time would tell, but if either team gets to the Champions League final it puts them in that conversation. They are definitely two of the top sides in Europe.

"It is up there for me, personally. Look in Spain; no team is close to Barcelona. In France, Lyon beat PSG nine times out of 10.

"And look at the fine margins - it's a difference of a goal or a decision in each game."

Regardless of the debate about the size of the game, it will certainly play a huge role in deciding the English league title.

Arsenal and Chelsea have won all of the past six WSL titles between them - Chelsea earning five, including the previous three - and while Manchester United and City were both in form before Christmas, it would take a remarkable turnaround for the trophy to leave London.

"It is season defining," says Asante.

"If Chelsea beat Arsenal, they will feel in the driving seat, and vice versa. They both know a lot is at stake; the league is tight and it is difficult to win if you drop points, particularly against rivals next to you in the table.

"If Chelsea win this, they will be six points clear. That will be a huge buffer, having that cushion and knowing they have to chase you."

If Arsenal are to close the gap, they must do so without two of their key players, Beth Mead and Vivianne Miedema.

Mead and Miedema are set to miss the remainder of this season with anterior cruciate knee ligament injuries. Mead suffered hers towards the end of the 3-2 loss to United, with Arsenal's woes compounded by the Netherlands international suffering the same injury weeks later.

Arsenal have continued to impress even without the pair, however, and Asante says she has been hugely impressed with the team's mentality and resilience, which could be key against Chelsea.

She said: "It's a travesty that they have lost two key players, but even prior to that, players like [Caitlin] Foord and [Frida] Maanum have taken space and responsibility, especially in attacking play.

"It's a resilient team, that has come through. Getting back captain and leader [Leah] Williamson is key, Raffaele Souza has been a great signing.

"The issue is long term. They are going for all trophies, and manager cannot rotate as much as they would like, that's where the issues will come in. They trust the players they have, they all have an identity everyone is on board with. I spoke to Jen Beattie and she said it's best she has played in in terms of togetherness."

Chelsea, meanwhile, have been close to perfect since a 2-1 defeat by Liverpool on 18 September, their opening game of the WSL season. They have not lost a game in all competitions since, with Sam Kerr and Fran Kirby leading the way for them in the goal charts.

They have not suffered long-term injury woes to the same extent as Arsenal, and have had their squad boosted by the return of Melanie Leupolz following maternity leave. For Asante, that strength of squad could give them the edge.

"For Chelsea, they have always been a relentless team going forward," she said. "They have so many options off the bench who come in seamlessly into the side.

"What makes Chelsea dangerous is that they can hurt you with the ball or without it. They are so dangerous in counter attack, with those focal points of Kirby and Kerr.

"Their only weakness is themselves, individual errors - they play out from the back, a lot of teams fall short in those moments. Ann-Katrin Berger is an exceptional goalkeeper but she has made one or two mistakes of late."

Last season, only a point separated these two teams in the final standings for the title, and head-to-head in the WSL era, there is nothing to separate them. In 22 WSL meetings, both have won nine each and there have been four draws.

So, where will the game be won and lost?

"I always think games are won and lost in midfield," says Asante. "The midfield battle, the team which provides service to the players who cause the most damage. Erin Cuthbert to Kirby, [Guro] Reiten getting delivery into the right areas. Chelsea score a lot of goals in a similar way, if they do that they can win.

"I hope it is moments of magic rather than error - a moment of inspiration. Whenever we have had a derby it has been moments like that, like Sophie Ingle's volley in 2020. I hope it is one of those games."