Atlanta opened a weekend series at Wrigley Field against the Cubs with a 9-5 victory. If the Nationals fall to the Marlins tonight, the Braves would clinch the National League East.
Then again, the Nationals also happen to be the hottest of the contending teams, having won 11 of their past 13 games, and they have ace Jordan Zimmermann going for his 19th win of the season against the Marlins.
"It feels good, but then again, I'll trade all those wins in for a spot in the playoffs," Zimmermann said.
The Red Sox can clinch the AL East title with a win over the Blue Jays tonight. The Cardinals also could clinch at least a tie for a spot if they win at Milwaukee and the red-hot Nationals lose.
Detroit's Max Scherzer will be making a fifth attempt for his 20th win, and the Tigers have an AL Central title in their sights this weekend, taking a magic number of four over the Indians into their opener against the White Sox.
The A's are just as close to a second straight AL West title and continue their series against the Twins with a postseason spot in their grasp during their final home series of the season.
And then there are the races that aren't that close to a conclusion, and there will be a trio of weekend matchups that will make a difference.
The NL Central showdown at PNC Park between the host Pirates and the Reds -- Nos. 1 and 2 in the NL Wild Card race -- will feature a marquee pitching matchup, with Bucs lefty Francisco Liriano meeting right-hander Mat Latos. Pittsburgh is ahead by a game heading into what will be the first of six meetings in the last nine games between the two Central clubs.
And it'll be a pair of AL Wild Card showdowns, one in Tampa Bay and one in Kansas City. The Orioles enter their series against the Rays three games behind the shared lead for the two postseason spots, and the Royals take on the Rangers with a gap of three games separating them from their first postseason appearance since 1985.
"This is definitely what they wanted when they threw in another Wild Card," Orioles slugger Chris Davis said. "But we just have to go out there every day and focus on the nine innings in front of us."
The Indians again will have the potential to jump into the Wild Card spot as they host the Astros again, and the Yankees will try to stay in the hunt as they start up with the last team standing in 2012, the Giants.
Friday's key games to watch (all times ET)
Reds (Latos, 14-6) at Pirates (Liriano, 16-7), 7:05 p.m. Preview >
Astros (Oberholtzer, 4-3) at Indians (McAllister, 8-9), 7:05 p.m. Preview >
Marlins (Turner, 3-7) at Nationals (Zimmermann, 18-8), 7:05 p.m. Preview >
Giants (Lincecum, 10-13) at Yankees (Sabathia, 13-13), 7:05 p.m. Preview >
White Sox (Rienzo, 2-2) at Tigers (Scherzer, 19-3), 7:05 p.m. Preview >
Orioles (Hammel, 7-8) at Rays (Price, 8-8), 7:10 p.m. Preview >
Blue Jays (Rogers, 5-7) at Red Sox (Lester, 14-8), 7:10 p.m. Preview >
Cardinals (Miller, 14-9) at Brewers (Hellweg, 1-4), 8:10 p.m. Preview >
Rangers (Perez, 9-5) at Royals (Santana, 9-9), 8:10 p.m. Preview >
Twins (Albers, 2-3) at A's (Colon, 16-6), 10:05 p.m. Preview >
Dodgers (Volquez, 9-11) at Padres (Erlin, 2-3), 10:10 p.m. Preview >
If the postseason started today ...
Wild Card: Rays at Rangers
Division Series: Wild Card at Red Sox* | Tigers at A's
Wild Card: Reds at Pirates
Division Series: Wild Card at Braves | Dodgers* at Cardinals
* - clinched postseason spot
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.
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