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Of the 11 teams that have tried and failed to beat Thomas Tuchel's Chelsea, from Manchester United to Liverpool to Atletico Madrid, Championship side Barnsley are the team who tested them the most.
They managed 14 shots against Chelsea in the FA Cup - 10 inside the box, four from outside - with a specific gameplan to press Tuchel's side high up the pitch.
They blocked passes into the Chelsea midfield and so rigorous was their pressing, two of Barnsley's front three had to be replaced by fresh legs after an hour. It forced Chelsea into a back five with the Premier League visitors' wing-backs unable to get forward.
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© Provided by Daily MailKepa Arrizabalaga saves a Barnsley shot. They were very effective at pressing the Blues high
I was at Oakwell that night and, believe me, Tuchel was not a happy chap on that touchline. So effective were Barnsley that Chelsea had to make emergency changes at half-time. They ended up winning courtesy of their only shot on target - a second-half tap-in by Tammy Abraham.
Leeds United boss Marcelo Bielsa, as a tactician who loves the little details, will no doubt be aware of how his Yorkshire neighbours made life difficult for the west Londoners in the FA Cup last month.
Copying the high press of Valerien Ismael's Barnsley may well be your best bet of disrupting Chelsea's play and inflicting a first defeat on Tuchel.
Another key factor at Oakwell was the poor state of the pitch, which didn't help Chelsea in their bid to play out from the back. The Elland Road pitch has not won any admirers this season either. Leeds reacted to the negativity by having their pitch relaid in January but it's still causing problems.
Chelsea will have to cope with that on Saturday, too, and that could play into the hands of a high-press tactic.
© Provided by Daily Mail The signature of Tuchel's Chelsea system has been to play with a back three and two midfield sitters. It makes the team compact and when in possession, the defenders can punch passes into the midfielders or find the wing-backs, who position themselves high and wide.
When Everton failed to press Chelsea during their Monday night defeat at Stamford Bridge, Andreas Christensen - at the heart of the back three - had time to bring the ball out and assess his options. Chelsea were able to dictate possession from there.
Mason Mount was rested but Callum Hudson-Odoi and Timo Werner played as inside forwards with the very effective Kai Havertz often moving deep into midfield to receive the ball in the build-up.
Chelsea recorded another clean sheet against Everton and have only conceded two goals in Tuchel's 11 games.
When under pressure, everybody gets behind the ball, bar one attacker. The wing-backs retreat to create a back five, with a protective shield sitting in front of the defence. Mateo Kovacic, Cesar Azpilicueta and Antonio Rudiger are the three outfield players to have made the most starts under Tuchel, which is intriguing given they weren't always in favour under Frank Lampard.
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Rudiger, in particular, looks a man on a mission to use this second chance to resurrect his Chelsea career.
What has stood out to me when watching Tuchel's Chelsea is how much more aggressive they have become.
© Provided by Daily MailAntonio Rudiger (right) looks desperate to make the most of his second chance at Chelsea
All over the pitch, they are looking more tenacious, with the back three shutting the door much more aggressively than they were previously.
They are winning more duels and getting good numbers around the ball.
The statistics show there has been a huge swing in the goals conceded column when comparing Chelsea under Tuchel to Lampard - they're conceding 0.2 per game now compared to 0.9 previously - but also they are not scoring as freely.
We're yet to see them truly let loose in attack because they've been focusing on making themselves difficult to beat, first and foremost.
Five of Tuchel's eight wins with Chelsea have been by a single goal, including four 1-0s, so they need to find a way to score more than they are.
Set-pieces will be key on Saturday. Chelsea have scored the most goals from set plays in the Premier League this season while Leeds, who go man-for-man, have conceded the most.
© Provided by Daily MailChelsea have become much more aggressive under Thomas Tuchel but less free-scoring
Significantly, after Monday's defeat at West Ham United, Bielsa publicly accepted the blame for the alarming amount of goals his side have been shipping from set-pieces.
I'm sure he will have been agonising over his long-held beliefs on man-marking from restarts. They even man-mark from throw-ins which, given there are 40 of those a game on average, can be used to drag you out of shape if your opponents are clever.
Top coaches adapt and it will be intriguing to see if Bielsa changes things for the visit of Chelsea on Saturday.
Leeds had two goals disallowed inside the first 10 minutes at the London Stadium but instead found themselves 2-0 down by half-time. That almost summed up their season. Both West Ham goals were avoidable.
The opener was a result of centre back Liam Cooper playing an over-ambitious diagonal ball when they had multiple men forward. Leeds lost possession and suddenly the entire team had to turn and retreat.
© Provided by Daily MailSet pieces will be crucial - Chelsea have scored the most, while Leeds have conceded the most
Right back Luke Ayling ended up sprinting back and sticking out a tired leg on Jesse Lingard to concede a penalty.
West Ham's second goal saw Craig Dawson easily lose his marker Diego Llorente and make a straight run to head home from a corner.
This is why I'd always recommend a combination of man-marking and zonal marking.
If Leeds mark as flimsily as they did against West Ham, Chelsea could clean up.
Tuchel has already got the better of Jurgen Klopp, Jose Mourinho, Diego Simeone and Carlo Ancelotti since taking charge in January, with none of their teams able to score against Chelsea.
Bielsa's reputation as a coach is up there with all of those names. The Leeds boss will be determined to do what they couldn't and make a dent in Tuchel's defence today.
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