No two Premier League clubs have endured more unexpected fortunes this season than Leicester City and Chelsea.
Despite having to pull off one of the greatest escapes in Premier League history to ensure their top-flight survival last term, the Foxes now find themselves two points ahead of the pack at the division's summit.
Chelsea, meanwhile, are currently the reigning champions, but retaining that title now seems impossible amid arguably the most unprecedented decline the Premier League has ever witnessed. They're all the way down in 14th place, just two points above the relegation zone and a whopping 14 away from a Champions League spot.
But many feel Leicester's bubble must burst eventually, whilst Chelsea will surely return to the dominant displays of last season sooner or later. Facing each other on Monday night, the 8pm kick-off at the King Power Stadium could well be the turning point in their respective campaigns.
That, however, will depend on which side dominates the key individual battles. With that in mind, here are FIVE Football Fancast view as the most important.
JAMIE VARDY VS JOHN TERRY
It's not been a particularly easy season for John Terry and things could get even tougher for the experienced centre-back on Monday night, when he faces the Premier League's most potent goal threat in Jamie Vardy.
The England international has bagged 14 goals in 15 appearances for Leicester City this season, setting a new Premier League record of netting in 11 consecutive fixtures in the process, but it's his lethality on the counter-attack that will concern the Chelsea skipper most.
Vardy's relentless energy and power-running on the break has been the predominant source of his Premier League-best tally and we've already seen Terry come unstuck against speedy strikers on several occasions this season - the 3-0 defeat to Manchester City in September, in which he was substituted at half-time, being the most notable example.
Kurt Zouma offers great pace and power on the cover, but if the 34-year-old is left isolated he'll have little chance of keeping up with Vardy. That could well see Jose Mourinho drop Terry for the more athletic and agile Gary Cahill.
DIEGO COSTA VS ROBERT HUTH
Almost a reverse of the aforementioned battle but under particularly different circumstances. Whilst Vardy has dazzled his way to the top of the goal charts and the Premier League's record books this sason, Chelsea frontman Diego Costa has scored just four goals across all competitions and only three domestically.
Meanwhile, although Leicester City's defensive record isn't exactly outstanding, centre-back Robert Huth has impacted the Foxes' results immeasurably since initially signing on loan last January, losing just five of the 29 league fixtures the former German international has been involved in.
A former Blue and a particularly physical defender, measuring in at a lofty 6 foot 3, Huth will relish the challenge of facing the ever-aggressive Costa on Monday night.
That being said, the Spaniard produced an impressive performance against FC Porto in midweek, the only missing ingredient being a goal to reward an evening of relentless industriousness, and will enter the Leicester clash with far more confidence than in recent weeks.
WILLIAN VS CHRISTIAN FUCHS
Despite Christian Fuchs' rather humorous name, his performances since joining Leicester City in the summer have been anything but comical, providing Champions League experience and steely influence on the left-hand side of defence.
Eden Hazard is unanimously viewed as Chelsea's most important player going forward but it's Willian who has proved the bigger influence this season, claiming two goals and one assist in 14 appearances whilst averaging 2.1 created chances and 1.8 successful dribbles per match.
Furthermore, it could be the Brazilian who has the biggest impact on Monday night. The likelihood is that Chelsea will sit deep to try and limit space for Leicester City on the counter-attack, making Willian's renowned speed and dribbling ability vital to getting the Blues up the pitch.
No doubt, however, Fuchs is a tried and trusted performer at European and international level. He won't concede space easily and also boasts impressive averages this year - namely 2.9 tackles, 3.6 interceptions and 3.7 clearances per match.
N'GOLO KANTE VS NEMANJA MATIC
As deep-lying midfield enforcers, N'Golo Kante and Nemanja Matic won't necessarily be going toe-to-toe on Monday night - but their roles could prove crucial for their respective sides.
The Serbian international will be required to try and sniff out those scintillating Leicester City counter-attacks before they happen, whilst summer signing Kante will attempt to contain some of the most talented creators in the Premier League - namely Eden Hazard, Oscar, Willian and Cesc Fabregas.
Regardless of Chelsea's recent form, that's no easy feat, but it could prove to be a considerably easier day at the office for the Frenchman than his Chelsea counter-part.
Indeed, he's averaged a whopping 3.8 tackles and 4.3 interceptions per match this season and is one of the most energetic midfielders on show in the top flight. Matic, meanwhile, although supremely talented, is struggling for form and lacks the natural pace to keep up with Vardy and Mahrez on the counter.
Resultantly, don't be too surprised to see Ramires line up alongside the 6 foot 4 midfield monolith on Monday night.
RIYAD MAHREZ VS CESAR AZPILICUETA
Our stats-mad mates at Whoscored rate Riyad Mahrez as the second-best performer in Europe this season with an average rating of 8.33 and Football Fancast feel inclined to agree.
The Algerian winger is third in the Premier League's goal charts with ten, second in the assist rankings with six, eighth in terms of created chances per match with 2.2 and top of the dribble table with 3.6 successful take-ons per match.
But if any Premier League full-back can nullify the 24-year-old's relentless attacking threat, it's probably Cesar Azpilicueta.
Even amid their current plight, the Spaniard has been one of Chelsea's more consistent performers this season, averaging 3.3 tackles and 2.6 interceptions per match, and his defensive technique is second to none in the English top flight.
Then again, Mahrez will enter Monday's game on the back of netting a hat-trick against Swansea City last weekend and his confidence is at an all-time high.