Manager Joe Maddon tips his cap to Chicago Cubs fans at Wrigley Field. Getty

The Chicago Cubs are heading to the 2015 National League postseason, while the World Series champions are on the cusp of elimination. It's the Cubs' first National League playoff berth since 2008, and it comes in the fourth season under former Boston Red Sox general manager Theo Epstein and in the first year under manager Joe Maddon.

The Cubs, who reached the postseason with the Oakland A's win over the San Francisco Giants on Friday night, are assured no worse than playing in the NL wild-card game on Oct. 7, and are expected to face the Pittsburgh Pirates who have a 5.5-game lead. The Cubs could use some momentum against the Pirates. After taking three out of four earlier this month at PNC Park, the Cubs have dropped the last two games at Wrigley Field.

Overall, it's been a rather surprising season for the Cubs. According to Baseball Prospectus PECOTA projections, the Cubs were projected to win 82 games. They have already won 89 games and should add to that win total with upcoming games against the lowly Cincinnati Reds and Milwaukee Brewers.

After a strong 2014 season, Jake Arrieta has put on a Cy Young performance this season with a 1.88 earned-run average in 216 innings. Left-handed starter Jon Lester, who arrived as a free agent in the December, has posted positive numbers as well, with 20 quality starts and a 3.43 ERA. Closer Hector Rondon followed up his breakout 2014 season with an even better 2015, earning 28 saves on a 1.74 ERA. Overall, the Cubs have the fifth-best ERA (3.51) in baseball. Last season, Chicago finished with the No. 21 ERA.

The rookie bats have been a big reason the Cubs have overachieved. Third baseman Kris Bryant has hit 26 homers and batted .280, and looks like a favorite to win the Rookie of the Year award. Meanwhile, middle infielder Addison Russell has provided a lift with 13 homers and 26 doubles. The future looks bright in Wrigleyville, as Bryant is 23 years old, while Russell is 21.

But the Cubs' bats have really come alive because of first baseman Anthony Rizzo. The 26-year-old picked up where he left off last season by ripping 30 homers and setting a career high in RBI (95). He has even stolen 17 bases, which is more than he stole in his four previous seasons combined. Rizzo is not expected to win the MVP, but it wouldn't be surprising if he finishes in the top five in voting.

The Cubs' success has come at the expense of the Giants. In the middle of August, the defending World Series champions looked like they had a decent chance to overtake the NL West division leaders Los Angeles Dodgers, or at the very least make a push for one of the wild card spots. But Bruce Bochy's squad faltered with a seven-game losing streak that began in late August, and failed to win all eight of the games decided by one-run in September. The Giants are eight games behind the Dodgers in the NL West with nine games remaining.