If you'd offered Huddersfield 12th place near Christmas and a win over Manchester United, most Terriers fans would of ripped your arm off. David Wagner's side come into this game full of confidence following Saturday's home win against Brighton and will feel a second famous scalp is a possibility.
As for Chelsea, a shock defeat at struggling West Ham left them 11 points adrift of Man City and will know they are in for a tough night in Yorkshire. Antonio Conte's side have a point to prove and a victory is crucial if they are to stand any chance of making up ground on the Manchester clubs.
Here's all you need to know before Tuesday's clash:
Tuesday night's meeting will be the first between the sides since the 2007/08 FA Cup, which was so long ago that Scott Sinclair was an 18-year-old prospect at Stamford Bridge and Avram Grant was the manager.
Huddersfield, then in League One, came to West London for the fifth round tie and trailed after 18 minutes when Frank Lampard superbly drove home the opening goal. The lower league visitors worked hard and got their reward on the stroke of half-time when Michael Collins smashed home a well taken equaliser.
After the break, Chelsea asserted their Premier League authority as despite having a Soloman Kalou goal ruled out, Lampard slid home his second from a rebound before Kalou did officially get his name on the scoresheet to gives the Blues a 3-1 win and sent them into the quarter-finals, where they were knocked out by Championship side Barnsley.
Huddersfield are expected to name the exact same team that beat Brighton 2-0 on Saturday in a 4-2-3-1 formation. Midfielder Phillip Billing and defensive duo Michael Kefele and Jon Gorenc-Stankovic are the only definite absentees for the home side.
Chelsea have no injury concerns, but may well change things up after a lacklustre performance in Saturday's 1-0 defeat at West Ham. Tiemoue Bakayoko endured another difficult day and was subbed off at half-time, meaning the Frenchman could be dropped in favour of Victor Moses or possibly Pedro.
Gary Cahill is also likely to return to the bench after a mediocre display in Stratford, with Antonio Rudiger likely to come in. Willian could also return to the starting eleven in a wing role after repeated impressive cameos from the bench in the league.
Potential Huddersfield Starting XI: Jossl, Smith, Schindler, Jorgensen, Lowe, Mooy, Hogg, Kachunga, Ince, Quaner, Mounie.
Potential Chelsea Starting XI: Courtois, Azpilicueta, Christensen, Rudiger, Moses, Fabregas, Kante, Alonso, Willian, Morata, Hazard.
A lot has changed since that FA Cup tie in 2008, and Huddersfield, especially at home, are not a side to be taken lightly. Chelsea know they need three points to stand a chance of challenging for the title and must play much better than they did at the London Stadium to get anything from this one.
Both sides offer immense creative talent as well as a strong physical presence, but the game looks likely to be a close one that could be decided by one of any key mini battles on the pitch.
Huddersfield have already beaten Man United at home this season and will relish the thought of the champions coming to the John Smith's Stadium. Both sides will offer a lot all over the pitch and if the likes of Hazard, Willian, Morata and Alonso are tamed as much as possible, an excellent draw could be on the cards for Huddersfield.
Prediction: Huddersfield 1 - 1 Chelsea