Less margin for error is the simple reason a few members of the U.S. Soccer staff fear the semifinal phase of World Cup qualifying the national team is about to enter more so than the final, definitive Hexagonal.

With six matches instead of 10, each game’s importance is magnified, as is the cost of a stinker. The math should be the only troubling aspect of this semifinal round, since the two games against Antigua & Barbuda can be taken as gimmees and the Americans have never lost a qualifier against either Jamaica or Guatemala.

Naturally, that last item prompts pondering the possibility that at some point one of those nations will beat the USA in a real match, and given the weird bounces and bizarre officiating that can crop up in Concacaf events, if style points have to be sacrificed for standings points, that’s the way to play.

Those points extracted out of Kingston, Guatemala City and Mazatenango over the past two dozen years didn’t come easily. So while some fans may regard the semifinal round as a walk-through to the Hexagonal as Coach Jurgen Klinsmann and his players continue to refine their relationships, they should remember what transpired three months ago, when the U.S under-23s failed to advance out of group play in the Concacaf qualifying tournament. And all of those games were played at home.

The points are all that matters, yet in the first competitive interval of the Klinsmann era, these subplots are to be monitored:

Center-back shuffle. The short-term issue is who will be paired in central defense with captain Carlos Bocanegra; the longer-term question is who might replace the captain adequately so the USA doesn’t need to rely on a 35-year old, as he will be in 2014.

Clarence Goodson has shown the best of three players to partner Bocanegra in the past few games, though a savvier, speedier alternative in Michael Parkhurst could be incorporated. Geoff Cameron is a hybrid of the two – Goodson’s size and Parkhurst’s ball skills – yet he doesn’t yet look as confident under pressure as either of those two, which isn’t surprising, given he’s still mastering the nuances of converting from midfield.

Further down the road are the fates of Tim Ream and Omar Gonzalez, who are already falling behind the rest of the field.

Left backs limping. Another omission in addition to Ream, Eric Lichaj, may resurface at the left-back slot if injuries dog Fabian Johnson, who suffered a calf strain warming up for the Canada game and was scratched. Edgar Castillo, who may be able to suffice during the semifinal phase as an emergency backup, suffered an injury in Thursday's training and like Johnson is doubtful for Friday's game.

The emergence of Johnson at left back, where he played for German club Hoffenheim during the second half of the Bundesliga season, gives Klinsmann an automatic starter in a position that was a headache for predecessor Bob Bradley. His solid defensive work is complemented by eagerness to get forward and engage in combination play. He can also cross with either foot, a factor that should boost the U.S. attack as he gets more time in the team.

While promising to imbue a more fluid, skillful style of play, Klinsmann has also stressed the importance of a strong foundation in the back. Breakdowns, both collectively and individually, have hampered the team in its recent games.

Midfield matters. Johnson’s value extends beyond his comfort on the ball and defensive acumen; his overlapping abilities and range enhance the abilities of Jose Torres, who started the play that led to the first U.S. goal against Scotland by winning a tackle near the sideline and releasing the ball.

Backed by Johnson and supported centrally by whichever teammate is available, Torres can find spaces and seams to supply balls into the attacking third. Torres at left mid is the antithesis of a romping, slashing Brek Shea, and switching one for the other significantly alters the midfield.

Yet the midfield is crowded with candidates, with Klinsmann dependent on the two-way attributes of Michael Bradley and whomever partners him in the middle. Jermaine Jones can contribute offensively – he assisted on two goals and scored once against Scotland – yet he’s also reckless. His muscle and experience are desired commodities but a player easily goaded into cautions can be detrimental. The minutes and opportunities for Maurice Edu are hard to ascertain.

Big guns. The much-ballyhooed debut of the Clint Dempsey-Landon Donovan duo didn’t dazzle in the 0-0 tie with Canada, and having both of them available is both a pleasure and a poser for Klinsmann. They are both creators and finishers and if on their game can handle the bulk of the playmaking given enough midfield support.

Whether the alignment is designated as a 4-4-1-1 or 4-3-3, getting Dempsey into a hole behind the forward(s) is probably his best spot, but working out the other positions has been tricky. Klinsmann can use a pair of central players behind Dempsey as the attacking mid with Donovan on one of the wings, which would keep Torres in the mix on the left in a 4-2-3-1. Torres and Donovan are very different players, so by using them on opposite flanks while allowing some freedom to roam, Klinsmann can confound both opponents and observers.

While many fans and pundits have speculated about which is the best formation to play, Klinsmann has stressed maximizing the abilities of players and finding the alignment that best fits their strengths and his tactics. While he’s used all four of his forwards on his 23-man roster the past three games he’s rarely put more than one on the field at the same time.  

He could opt for the size and power of Jozy Altidore, or might prefer the guile and pace of Herculez Gomez.

In the opener Friday against Antigua and Barbuda, there will be pressure to attack early and score often. Four days later, Klinsmann gets his first test in a hostile Central American venue against opponents who have ample experience against American players at the club and international levels.

Six points would be great, four points are essential. The math starts here.