NBA's 3 unbeaten teams doing it with defense

At this point last season, the defense-first Indiana Pacers stood as the league's lone undefeated team. The Golden State Warriors, Memphis Grizzlies and Houston Rockets still stand undefeated this season, and they're following the Pacers' means of dominance from a year ago.

The Grizzlies are a defensive-minded outfit, sure, but the Rockets employ James Harden - not a great defender - and the Warriors are considered an offensive juggernaut, first and foremost. Memphis has been mediocre offensively, while Houston and Golden State have excelled, but all three have succeeded in smothering the opposition to an incredible degree.

The table below shows each team's offensive and defensive rating, which is simply how many points per 100 possessions a team scores and allows, and its league rank in each category.

The league's three best defenses are undefeated. This shouldn't be surprising, especially with so few games played; extreme performances are magnified in small sample sizes. 

Their performances become more impressive, though, when the opponents they've played are considered. All three teams play in the superior Western Conference, and of their 15 collective games, just five have come against Eastern Conference teams. Basketball Reference adjusts offensive and defensive rating for the quality of a team's opponent, and doing so makes these teams look even stronger. 

Each team was expected to be strong on defense based on reputation. 

The Rockets employ Dwight Howard, one of the league's premier rim protectors, and strong perimeter defenders in Patrick Beverley and the newly-acquired Trevor Ariza. Last year, they ranked 12th in team defense, despite Harden's oft-maligned defensive presence. He's been noticeably better to start the season, especially in transition, and seems to have embraced that he'll need to improve in that regard for the team to take a step forward.

In terms of specifics, the Rockets aren't necessarily doing any one thing very well. They afford opponents a fairly average shot mix, foul at a roughly average rate and have succeeded largely by getting stops. Houston's opponents are sporting a 43.1 percent effective field goal percentage (which accounts for the value of 3-pointers), better than all teams except for the Chicago Bulls. That mark is unsustainable over a full season, but it speaks to the group's effectiveness early on.

The Grizzlies also rank well in eFG% allowed, but their dominance comes primarily at the rim, where no team forces opponents to shoot a worse percentage. Because of Marc Gasol's presence inside, and strong perimeter defenders who can funnel ball-handlers exactly where coach Dave Joerger's system dictates, opponents are shooting just 49 percent at the rim against Memphis.

The Grizzlies also force turnovers on nearly 20 percent of opponent possessions, the fifth-best mark in the league. Along with strong defensive rebounding and an acceptable foul rate, the Grizzlies are essentially shutting opponents down in all facets of statistician Dean Oliver's Four Factors of Basketball Success. 

The Warriors pose the most interesting case. They're considerably better than Memphis and Houston according to the early numbers, and they had already established a strong reputation as 2013-14's No. 3 defense.

Golden State is, on paper, an offensive force, yet the Warriors were only the league's 12th-best offense a year ago. Given the presence of Stephen Curry and David Lee, it was widely assumed this core would figure out offense first and defense second. That hasn't been the case, though, and with their offense looking more creative and deadly under new head coach Steve Kerr, opposing teams in the West should be terrified.

The Warriors are the only team in the league that can reasonably expect to be a top-five outfit on both ends of the floor, a standing that would leave them in elite company.

Memphis, Houston and Golden State can't maintain this level of defensive dominance. It's simply too difficult to stick at such an extreme pole over 82 games, and no team has allowed fewer than 100 points per 100 possessions over an 82-game season since the 2004-05 San Antonio Spurs. 

But all three teams have shined defensively against tough competition, which should serve warning to the rest of the league that the ceiling for these clubs is the league's best defense.