Off to its best start since 1981, San Diego State faces yet another stiff test with giant-killer Northern Illinois coming to town.

The No. 19-ranked Aztecs (4-0) are looking to win five straight games to open a season for the first time since 1975 and only the second time in 57 years. San Diego State took a step toward that goal by defeating Air Force 28-24 in its Mountain West Conference opener. Rashaad Penny led the way, amassing 128 yards and three scores on 20 carries. Penny became the 14th player in school history to eclipse 2,000 career rushing yards.

Northern Illinois (2-1) is more than capable of giving the Aztecs a battle. The Huskies opened some eyes around the nation with a 21-17 upset victory over Nebraska two weeks ago. Northern Illinois became the first-ever Mid-American Conference (MAC) team to defeat the Cornhuskers after Shawun Lurry and Jawuan Johnson returned a pair of first-quarter interceptions for touchdowns.

San Diego State is 5-0 all time against Northern Illinois, most recently beating the Huskies 42-28 last season. The Aztecs' wins in the series have come by an average margin of 25.2 points.

Northern Illinois at San Diego State

Kickoff: Saturday, Sept. 30 at 10:30 p.m. ET

TV Channel: CBS Sports Network

Spread: San Diego State -11

Three Things to Watch

1. Christian Chapman is a model of consistency

Northern Illinois is no stranger to the problems that Aztecs QB Christian Chapman can create for an opposing defense. Chapman played a key role in San Diego State's 42-28 win over the Huskies last season. He threw a career-high three touchdown passes and totaled 126 yards on 11-of-19 passing.

Chapman has only grown better this season and is 17-3 as the starting quarterback over his career. His accuracy has given the Aztecs a steady hand at the helm in the passing game. San Diego State is 9-0 when Chapman posts a passer rating of at least 170.

The junior did not throw an interception against Air Force. He has gone three games and thrown 68 straight passes without tossing an interception. For his career, Chapman has totaled just eight interceptions against 27 touchdown passes. He leads Mountain West quarterbacks with a 149.93 quarterback rating and a 66.7 completion percentage.

2. Huskies' defense is rising up

It's possible that Northern Illinois could end up being one of the better all-around defensive teams in the nation this season. Through three games, the Huskies lead the MAC in both total defense (303.3 ypg) and rushing defense (97.67 ypg). They also rank second in both scoring defense (16.7 ppg) and interceptions (6).

Northern Illinois has made definite strides from last season in keeping offenses on their heels. That progress was most evident against Nebraska. The Huskies put tons of pressure on Cornhuskers quarterback Tanner Lee, totaling three sacks and nine tackles for a loss and turning a pair of Lee interceptions into pick-sixes. Sutton Smith led the way with four TFL. Three other Northern Illinois players tallied 10 or more total tackles — Bobby Jones (14), Kyle Pugh (11) and Mykelti Williams (10).

The Huskies will need to get defensive stops early and often again this week. San Diego State has a knack for mounting long, soul-crushing drives on opponents. The Aztecs lead the Mountain West and rank seventh nationally in time of possession (35:01).

3. Aztecs conquer the first half

Teams have a tough time getting off to strong starts against San Diego State. The Aztecs have outscored opponents 61-33 during the first half this year. This includes a 44-17 advantage in the second quarter. They have trailed at halftime only three times in their last 28 games.

San Diego State is doing it with defense. The Aztecs have held opponents scoreless on their opening drive 24 times in the last 32 games dating back to 2015. That initial momentum builds as games progress. Against Air Force, for example, San Diego State allowed 154 total yards on 48 plays over the final three quarters after giving up 99 yards on 18 plays in the first quarter.

Currently, the Aztecs rank second in the Mountain West in scoring defense (19.5 ppg) and total defense (292.3 ypg) — trailing only the Falcons. They are allowing just 13.8 first downs per game, the fewest of any team in the league.

Final Analysis

Northern Illinois is probably the toughest remaining regular-season opponent San Diego State will face. The Huskies have the tools on defense to hang with the Aztecs for four quarters. Still, San Diego State has a legitimate Heisman Trophy candidate in Rashaad Penny powering its offense and a tough defense on top of that. It's too much to expect Northern Illinois to engineer another major upset.

Prediction: San Diego State 27, Northern Illinois 20

— Written by John Coon, who is part of the Athlon Contributor Network. Coon has more than a decade of experience covering sports for different publications and outlets, including The Associated Press, Salt Lake Tribune, ESPN, Deseret News, MaxPreps, Yahoo! Sports and many others. Follow him on Twitter @johncoonsports.

(Top photo courtesy of San Diego State University Athletics Facebook page; Shawun Lurry photo courtesy of @NIUAthletics)