Baseball's postseason races checked in early with games under the September sun on Saturday, and while they'll keep churning into the night, a trio of daytime meetings already stirred things up pretty well.

As the Pulse of the Postseason got racing for another day, the Reds put their collective foot down as the No. 2 National League Wild Card entry, while the Red Sox and A's each took another step closer toward a division title.

With the American League Wild Card race still in beautiful disarray, featuring six teams within 4 1/2 games of each other entering Saturday's slate, night games from coast to coast will keep shuffling teams in that race and others, with NL leaders Atlanta and Los Angeles both hoping to narrow their magic numbers.

On a Saturday afternoon in three cities where the postseason races were front and center, three teams took steps forward in their quest, while three took steps back.

In Boston, the Red Sox knocked their magic number down to six pending the Rays' night game at Minnesota, getting ahead of CC Sabathia and the Yankees and holding on for a 5-1 victory that does as much damage to their rivals as it helps their own cause. The Yankees are 2 1/2 back for the second AL Wild Card spot and the Orioles -- who fell to the Blue Jays, 4-3 -- are three back.

Jon Lester was masterful in the victory for the Red Sox, going eight innings while allowing one run on three hits, and Boston will be eyeing a sweep in its Sunday night finale against the Yanks.

For Yankees manager Joe Girardi, there's no denying the importance of Sunday's game, whatever label one might use about it.

"It's only a must-win game when it's an elimination game, but I think it's a real important game," Girardi said.

In Milwaukee, the Reds were up against the Brewers, out of the race but winners of the opener of the weekend series. Cincinnati scored one more run in the first inning than it scored the night before, thanks to Shin-Soo Choo's leadoff double, and that was just the beginning for Choo on this day. He also had a two-run homer, and he added a sacrifice fly for the team's seventh run in a 7-2 win after scoring on Joey Votto's mammoth blast in the sixth.

The win stemmed the tide of the Nationals' recent rush toward the Reds, their lead in the NL Wild Card down to 4 1/2 games entering Saturday's games and sure not to shrink as the Nationals take on the Phillies at night.

In Texas, the concept of scoring early continued, as the A's got on the board in the first inning when Brandon Moss doubled off Yu Darvish, sending Josh Donaldson sprawling across the plate for a 1-0 lead. With Darvish and Bartolo Colon going toe-to-toe and Grant Balfour picking up the save, that was the final score -- not exactly a typical outcome for a day game at Rangers Ballpark in Arlington.

But, then, these two pitchers were anything but typical. Other than the first inning, Darvish shut down what has been one of the most explosive offenses in the game of late. But Colon topped him not only by dancing out of trouble every time but by doing some glove work, as well, starting a nifty 1-4-3 double play in the third. Whatever the recipe, it was another big one for the A's, winners of eight of 11 and 12 of 15.

That was Darvish's fourth 1-0 loss of the season, becoming the first Major League pitcher with that dubious distinction since Orel Hershiser in 1989.

The A's are now 5 1/2 games ahead of the Rangers in the AL West, with just 14 games to play. For reference, the Rangers led the A's by three games with 14 games to play last year, but there were six more meetings between the two teams -- not just one, Sunday's.

"This is the position you fight to put yourself in all year," said Moss. "Now we have to finish it."

All due respect to Girardi's opinion, but the Rangers' Elvis Andrus puts Sunday's finale -- and, well, the rest of the schedule -- in a different category.

"Every game right now is a must-win situation," Andrus said. "That's the way we have to face it right now and not take anything for granted. We have to keep fighting and take care of business."

The Orioles, meanwhile, did not help themselves in Toronto, falling, 4-3, to the Blue Jays, with closer Casey Janssen capping off the game with a fielding gem to nail Manny Machado, who earlier had become the youngest player in Major League history to hit 50 doubles in a season.

As Saturday's time under the sun headed toward games under the lights, plenty more will be decided by the time another September day is through.

Under the lights, the Pirates host the Cubs and the Cardinals host the Mariners as the NL Central story continues to evolve. The Rays play at Minnesota with the AL Wild Card position hanging in the balance, while the Royals meet the Tigers and the Indians face the White Sox in hopes of keeping pace. And magic numbers are on the minds of the Braves (six) as they meet the Padres and the Dodgers (four) as they meet the Giants.

Saturday night's key games to watch (all times ET)

Cubs (Baker, 0-0) at Pirates (Cole, 7-7), 7:05 p.m. Preview >
At least it's not Friday the 13th anymore. The Pirates have lost their last 11 on that freaky day now.

Phillies (Hamels, 7-13) at Nationals (Gonzalez, 10-6), 7:05 p.m. Preview >
Time is of the essence, but the Nationals have picked a good time to be the hottest team around.

Royals (Santana, 8-9) at Tigers (Fister, 12-8), 7:08 p.m. Preview >
The Tigers are 45-27 at home this season and have won 78 of the last 116 at Comerica Park.

Indians (Jimenez, 11-9) at White Sox (Rienzo, 2-1), 7:10 p.m. Preview >
The Tribe has the look of a team collecting itself on the road as the stretch run hits high gear.

Padres (Erlin, 2-2) at Braves (Medlen, 13-12), 7:10 p.m. Preview >
The Braves' magic number is stuck at six after Friday's loss and the Nationals' win.

Rays (Moore, 15-3) at Twins (Albers, 2-2), 7:10 p.m. Preview >
Pitching was the difference Friday night, and Moore steps in as the team's winningest pitcher.

Mariners (Paxton, 1-0) at Cardinals (Wacha, 3-0), 7:15 p.m. Preview >
The Cardinals found a way to win Friday, with a little help but a lot of making their own opportunities.

Rockies (Oswalt, 0-5) at D-backs (Miley, 9-10), 8:10 p.m. Preview >
The D-backs remain 12 1/2 out in the NL West and 9 1/2 games out in the NL Wild Card.

Giants (Lincecum, 9-13) at Dodgers (Nolasco, 13-9), 9:10 p.m. Preview >
The magic number is down to four for the Dodgers, which means they still could clinch the NL West this weekend.

If the postseason started today ...

As of Friday's games

AMERICAN LEAGUE

Wild Card: Rays at Rangers
Division Series: Wild Card at Red Sox | Tigers at A's

NATIONAL LEAGUE

Wild Card: Reds at Pirates
Division Series: Wild Card at Braves | Cardinals at Dodgers

Postseason 101

Magic numbers
To calculate a team's magic number, take the number of games it has remaining and add one. Then subtract the difference in the number of losses between that team and its closest pursuer.

Tiebreaker scenarios
A tiebreaker game will be played to determine a division winner, even if the tied clubs are assured of participating in the postseason. If a division championship tiebreaker is necessary, the head-to-head record between the clubs will determine home-field advantage. If the head-to-head record is tied, then division record will be the next tiebreaker.

If two clubs are tied for the two Wild Card berths, home-field advantage will be determined by the head-to-head record between the clubs. If the head-to-head record is tied, then division record will be the next tiebreaker.

Tiebreaker rules »

2013 postseason schedule

NL Wild Card Game: Oct. 1
AL Wild Card Game: Oct. 2
NL Division Series begin: Oct. 3
AL Division Series begin: Oct. 4
NL Championship Series begins: Oct. 11
AL Championship Series begins: Oct. 12
World Series begins: Oct. 23