Life is grand at Tottenham. On Sunday, Spurs smoked West Ham, 4-1. The win moved Mauricio Pochettino's side within four points of first place and two points out of fourth. The top four and a berth in the UEFA Champions League doesn't just look like a vague dream for them -- they're genuinely one of England's top teams.

And this has been no run of beating up the weaklings, but struggling in the big matches. Two weeks ago, they traveled to Emirates Stadium and had the better of most of the match against Arsenal in open play, but came away with only a draw, having struggled to stop Arsenal crossing into the box. In September, they hosted the EPL's other top side, Manchester City. Spurs won that match going away, 4-1. By expected goals, a measure of the quality of chances created, Tottenham had the better of two matches against the EPL's two best sides overall.

This is not just two good matches either. They've played like this against the full swath of league opposition, from title contenders on down. They have the EPL's second-best goal difference (plus-13) and the third-best expected goals difference (plus-10). This is the record of a top-four club.

If you only looked at the club's table position, Spurs' run might seem relatively unremarkable. Last year Tottenham finished fifth, and they're fifth again after 13 matches. But what's more important is how they got there. In 2014-2015, Spurs were a defensive shambles, who stole results on clutch late goals rather than controlling matches. They conceded 223 shots from the danger zone, the region in the center of the box from which most goals are scored.

Those numbers place them fifth-worst in the EPL, and two of the teams below Tottenham Hotspur (Burnley and QPR) were relegated. This inability to prevent chances in dangerous areas led to Spurs conceding 53 goals and 51 expected goals, both numbers solidly in the bottom half of the league.

But this year has seen a radical turnaround in Tottenham's underlying defensive numbers. Now Spurs stand near the top of the league in every major defensive statistic, and they trail only Manchester United and Arsenal in expected goals conceded.

So what happened?

While Pochettino has had more time to teach his pressing principles, the biggest change is surely one of personnel. Chairman Daniel Levy and his head of recruitment Paul Mitchell picked up a new center back in Toby Alderweireld and a young central midfielder Dele Alli. Pochettino saw the making of a quality defensive midfielder in center back Eric Dier. Thanks to these moves, Spurs have made over their defense, and especially the midfield.

Last season the primary central midfield pair at White Hart Lane was Ryan Mason and Nabil Bentaleb, with Bentaleb playing defensive midfielder behind Mason in a more box-to-box role. This season the two have not played a single minute together in central midfield. Instead Dier has been a fixture at the defensive midfield position, while Alli and Mousa Dembélé have played more minutes than Mason or Bentaleb.

As various analysts showed last year, Mason and Bentaleb struggled mightily with their responsibilities in Pochettino's system. With Mason and Bentaleb in central midfield together, Spurs had a minus-six expected goals difference, compared to a nearly even xGD with other central midfield pairings. Their struggles to track runners through midfield or close down passing lanes led to a continuing surge of attacks into dangerous areas. And when runners got through, the back line was rarely up to the task of snuffing out the danger. Here is just one example (from an excellent tactical piece by Brett Rainbow) of how a missed assignment from Mason in midfield led to a good scoring chance for Spurs' opposition.

In this case the truly dangerous action is Davis' run on the ball after he receives the ball from Mane. He's able to carry the ball unchecked to the top of the 18-yard box. Davis has a possible passing lane to Graziano Pellè, as well, but center back Federico Fazio allows him to take on the chance himself. All of this happens because of the missed assignment in midfield overloading the defenders. This was the norm for Spurs last season, but it isn't happening anymore with Dier and Alderweireld in for Mason and Fazio.

To track the dangerous attacking actions that resulted from Spurs' defensive problems, I created an expected goals system for pass value. This tracks the likelihood of a goal being scored in the next seven seconds after a completed pass, based on the location of the pass, where it starts and where it ends, and what type of pass it is (headed/through ball/chipped in the air). The concept is the same as expected goals for chances created, but now applied to passes. I found that Tottenham have shut down the most dangerous opposition passes this year, mostly by preventing them time to work freely in the half-spaces just outside the box.

These are passes with an expected goals value of 0.07 or above. In the equivalent fixtures last year, Spurs were consistently vulnerable to passes from the half-spaces on both sides of the box, with 15 of the big passes coming from those zones. This year, with an overhauled midfield and an improved back line, those chances have dried up.

This defensive vulnerability popped up most notably in a series of blowout losses to Manchester City, Liverpool and Manchester United. Now Spurs have the quality and organization in defense to prevent even the best attacks from working into dangerous areas. Subjectively, Spurs have tightened up their chance prevention to a degree that is hard to believe in comparison to last year's troubles.

Such a radical shift in quality and results raises the question of whether the numbers really are unbelievable. When teams manage to massively improve their defensive quality, is it something that can be maintained?

Since 2010-2011 in the big four European leagues, this year's Spurs team have made the eighth-largest improvement in defensive quality. Spurs are clearly outliers in this year's Premier League, but other outliers similar to Spurs tended to do pretty well, even after their crazy starts to the season. Among the teams with the top 20 improvements, about two-thirds of the decrease in expected goals conceded was maintained.

These 20 teams averaged about 1.35 expected goals allowed per match in the previous season. Then they made a huge leap in the first 13 matches of the next season, down to just 0.88 expected goals conceded. Over the remainder of the season, these clubs averaged about 1.04 goals allowed per match. Teams that play excellent defense over the first 1,000-plus league minutes typically just are good defensive sides, regardless of how much they may have struggled in the past.

So, even if we knew nothing else about Tottenham, we should expect their improvement in preventing chances to be mostly maintained. Given that there is a clear reason for improvement -- new personnel in central midfield and defense -- it is all the more likely this is the real Tottenham Hotspur. By stepping up to fix the clear problems in last year's Spurs squad, Pochettino and the Spurs front office have built a true top four contender.