ST. PETERSBURG, Fla. -- The similarities between the Rays and Red Sox squads that will face off in the American League Championship Series are far-reaching. Both teams feature deep pitching staffs and lineups that emphasize grinding at-bats.

Yet one of the most important shared traits is among the least discussed and, in some ways, most unlikely. In recent years, both Tampa Bay and Boston were accused of far-reaching defensive shortcomings. Now, the two have emerged among the best fielding teams in baseball.

The about-face that has played a huge role in the two teams’ arrival in baseball’s Final Four. Two teams that had been accused at times of indifference to baseball are now succeeding with a blueprint that emphasizes glove work.

The Rays were particularly focused on their acute need to improve defensively.

“When we had our post-mortem on the ’07 season and talked about what we wanted to do in the upcoming winter,” said Rays Executive Vice President of Baseball Operations Andrew Friedman, "the big focus for us was on run prevention.

“We felt like that was our biggest Achilles heel in ’07 and an area that we could make the biggest improvement. So we focused on improving our defense and improving our pitching.”

The Rays accomplished that goal in spectacular fashion. A Tampa squad that gave up a major-league worst 944 runs in 2007 did a radical about-face, giving up 671 runs (tied for third fewest in baseball). That 273 run improvement was the third largest since 1900.

Had the team pursued a gut-and-renovate strategy with its pitching staff, it would have been easy to credit a host of new pitchers with the improvement. Yet of the 13 pitchers who logged at least 20 innings for the Rays this year, 10 were holdovers. Of those 10 pitchers who also pitched in Tampa in 2007, eight had significantly improved ERAs this season. (See chart.)

While the young pitching staff deserves some credit for its collective progress, there can be no question that improved glove work played a huge—perhaps even the largest—part of that improvement.

“This (Rays) team is phenomenal at defense. You couldn’t always say that, but they shored up their defense and look what happened,” noted Sox pitcher Paul Byrd. “People always said, ‘Their pitching is bad—they gave up all these runs.’ But they couldn’t play defense.

“I think the pitching’s turned the corner a little bit, but now all of a sudden they have great defense. Now, all of a sudden, they have great pitching.”

In 2007, the Rays often seemed a group of square pegs in round holes in the field. Their miscast defense featured the likes of Aki Iwamura at third, Brendan Harris at short, B.J. Upton at second, Delmon Young in center and Jonny Gomes in an outfield corner.

The result was a cover-your-eyes brand of baseball. According to both baseball talent evaluators and various statistical measures, the 2007 Devil Rays were one of the worst defensive teams of all time.

Statistics for defense remain the subject of widespread debate, and most of the progressive teams (including the Sox and Rays) that use their own defensive measures guard their statistical secrets more closely than the Da Vinci Code. Suffice it to say that many front offices consider error totals and fielding percentage largely irrelevant, since those statistics can reward teams that are sure-handed but with terrible range and ball-tracking skills.

Defensive efficiency is viewed as a better means of evaluating defense. The statistic defines how often a team turns a ball in play into an out. According to Baseball Prospectus, the 2007 Rays had a defensive efficiency rating (defined as the number of balls in play that the team turned into outs) of 65.6 percent.

That mark translated to the worst of any club in roughly a half-century of data. By some measures (including this interesting analysis), the team was more than 100 runs worse than a defensively average team.

Oh, how the world has changed. In 2008, Iwamura moved from third to second, a position where he is vastly superior. The team replaced the dismal Harris at short with an outstanding defender in Jason Bartlett. The arrival of excellent third baseman Evan Longoria at third solidified the infield, which already featured gifted glove man Carlos Pena at first.

Upton, meanwhile, relocated from the infield to center, where he combined with fellow flyer Carl Crawford to cover immense ground. The difference was undeniable.

“You go around their infield: Longoria, Bartlett, Pena, they're Gold Glove-caliber. They're good. Navarro is good. Their outfielders, the ball doesn't hit the turf very much,” said Sox manager Terry Francona. “Their defense, they may be the best in baseball. They're athletic. They catch the ball. The ball usually ends up where it's supposed to.”

This year, the Rays led the majors in defensive efficiency with a 71.0 percent rate of converting balls in play into outs. One executive suggested the Rays had one of the biggest single season defensive improvements in big-league history. Another, while acknowledging the challenges of measuring defense, said that the Rays might have improved by as many as 15 wins purely on the basis of improved work in the field.

While the Rays led baseball in defensive efficiency, the Sox finished fifth in the majors with a 69.9 percent conversion rate. A franchise that had been known in the recent past as a bunch of immobile mashers—Francona has admitted that in past seasons his team could be described as “plodding”—has transformed into a group that mixes athleticism with tremendous defensive instincts and positioning.

The outfield now features players with excellent instincts in Jason Bay and J.D. Drew in the corners. Center is occupied alternately by Coco Crisp and Jacoby Ellsbury, both of whom were rated by an American League coach as elite defenders.

Boston’s infield, meanwhile, featured Gold Glove corners in Mike Lowell and Kevin Youkilis for much of the season, and middle infielders who always seem to be in perfect position to make plays in Jed Lowrie and Dustin Pedroia.

The Sox teams of 2003 and the first half of 2004 often had to bash their opponents into submission. But since then—particularly in the 2007 and 2008 seasons, when the Sox featured one of baseball's most efficient defenses—the team shifted to emphasize run prevention through improved defense. The result was a team that was capable of finding ways to win even when David Ortiz was out of the lineup with injury and after the trade of Manny Ramirez.

“Every team that wins the World Series plays great defense,” said Sox infielder Alex Cora. “You can’t give the other team more than 27 outs. If you give the other team more than 27 outs with the way (playoff) teams swing the bats, other teams are going to take advantage.

“In ’04, I wasn’t here, but when they pulled that trade (of shortstop Nomar Garciaparra) for (first baseman Doug) Mientkiewicz and (shortstop Orlando) Cabrera, that’s what they were trying to do,” said Cora. “They were trying to get better on defense. When you pitch and play defense, you’ve got a shot. Even defense in basketball, defense in football, they win championships.”

The notion may seem exaggerated. Still, it is worth noting that the Sox made several decisions in the Division Series against the Angels—particularly related to the its infield corners—with defense as the top priority.

The team wanted to keep Mike Lowell in the lineup as much to man third base and keep Youkilis at first as it did because of his offensive potential. When Lowell could not play, the team opted for Mark Kotsay rather than Sean Casey in large part because Kotsay is the better defender. Such moves underscore how central the Sox now view their defense to their success.

“Pitching and defense, regardless of how much people say about our offense, it’s pitching and defense,” said Cora. “We got to a point in the season when we started turning more double plays, we started catching the ball better and we started playing better.”

The Rays have followed a similar course all season. The result is an ALCS clash of two teams that have quietly followed very similar defensive blueprints to reach this point. 

Alex Speier is a Senior Writer for WEEI.com.