You have to go back 20 years to 1996 for when the Hammers last finished above Chelsea in the table—it's only happened twice in the Premier League, the other time being 1993/94—and Slaven Bilic's side are well on their way to doing it this term.
Nine points separate Chelsea from West Ham ahead of the weekend's action, with the Hammers currently sitting in fifth while Chelsea linger further back in 10th place.
A big part of their differing fortunes this term has been the form of Dimitri Payet. Just as Chelsea's own flair players have struggled to make any sort of impact, Payet has been the standout performer in West Ham's attack.
The Frenchman has really caught the eye, being involved in 15 goals for his club in the league this season. Eight of them have been goals, with his trademark free-kicks adding to the threat he carries in open play.
It's no coincidence that West Ham's form suffered considerably when Payet was out injured earlier this season. Picking up an ankle injury against Everton in early November, we didn't see Payet back in action until the new year.
In those seven matches he missed, the Hammers won just once, drawing five. The dynamic Payet adds from midfield was dramatically missed, reinforced by the fact Bilic's men have now won eight of their 15 games in all competitions this year with Payet back fit.
To prevent them adding to that tally on Saturday, Chelsea really need to shut Payet down. That means John Obi Mikel can't afford to have a bad day.
Chelsea's Nigerian has been excellent under Guus Hiddink. He's revitalised his Stamford Bridge career since late December, putting in the sort of performances that strengthened his reputation in 2012 when Chelsea won the UEFA Champions League.
What Mikel has added to the Blues' midfield is a bit more steel—something that was missing for much of the campaign before he stepped in. He has marshalled opposition attackers, and that in turn has given Cesc Fabregas more licence to get forward and impact matches.
It's snowballed, with Diego Costa feeding off Fabregas' influence to rediscover his scoring touch.
Mikel has been essential in it all.
Whoever wins that midfield battle at Stamford Bridge on Saturday will most likely come out on the winning side. Can Payet dominate Mikel to open gaps elsewhere in the Chelsea defence? Or will Mikel shut him down like he has most others to give Chelsea the platform they need?
That's what it all comes down to as Chelsea's lineup can boast a stronger depth of talent from their goalkeeper through to the attack.
Costa will be missing through suspension, which is a big blow for the Blues, but if Chelsea can get the ball moving forward at pace, disrupting West Ham's attacking flow, they can cause them real damage.
In order to dominate the territorial battle, Payet can't be afforded the space he has had at times this term. We saw as recently as last weekend in the FA Cup against Manchester United just what a threat he can be.
United struggled to contain Payet and were fortunate to come away from the game earning a replay. The Hammers looked the more fluid of the sides, with the French talisman pulling the strings.
The free-kick Payet scored was a magical moment that added to the fine afternoon he had.
Payet isn't a selfish player, which shows in the high number of assists he has picked up. He's capable of scoring the wonder goals we have seen this season, but his threat is equally apparent when he's feeding others.
So much of what West Ham have achieved has been down to his brilliance. Had the likes of Eden Hazard rivalled his performances this term, the Blues wouldn't be in the position they find themselves now.
Chelsea's star man on Saturday has to be Mikel, though. Without him performing, it's going to be open season for Payet.