The second leg of Champions League knockout round matches begins today, so let's take a look at each team's chances of advancing to the quarterfinals. Using ESPN's Soccer Power Index (SPI) we calculated the offensive, defensive and overall rating of each team and used those to simulate each matchup 10,000 times. Here's a look at the four matches this week:

Arsenal -- AC Milan
(Milan leads 4-0)

If AC Milan can hold on to its 4-0 first-leg advantage, it would break a streak of three straight Round of 16 eliminations by English clubs. If Arsenal can pull off a comeback, it would become just the fourth team in the history of UEFA competition -- and first in Champions League play -- to overturn a four-goal first-leg deficit. The odds are clearly against Arsenal, which needs a minimum of four goals to advance, and SPI gives the Gunners a 6.5 percent chance of scoring four or more goals. Arsenal has scored four or more goals in a match four times this campaign, including a 5-2 win over Tottenham two weeks ago. Milan, however, has yet to concede more than three goals in a match this season.

Benfica -- Zenit St. Petersburg
(Zenit leads 3-2)

Benfica was undefeated in the 2011-12 campaign in both the Portuguese Liga and Champions League before falling 3-2 to Zenit St. Petersburg in the first leg. Benfica is winless since then, losing twice and drawing once while scoring just twice in three league matches. Zenit has played one match since the first leg, a draw against CSKA Moscow this past weekend, as the Russian Premier League resumed from its winter break. Benfica is looking for its first quarterfinals trip since the 2005-06 competition, while this is Zenit's first trip to the knockout stage. SPI gives Benfica the edge in the second leg, as the Portuguese side is 15-1-3 in its last 19 European home contests, but Zenit is still a slight favorite to advance to the quarters.

Apoel Nicosia -- Lyon
(Lyon leads 1-0)

Apoel has played well at home in European competition of late, as it had a nine-game undefeated streak at home snapped by Shakhtar in the final Group Stage match. The Cypriot side catches a Lyon squad that has struggled since winning the first leg 1-0, taking just one point from its last three Ligue 1 matches. Despite all this, SPI still sees Lyon as the favorite to advance, as Apoel scored one or zero goals in 63.3 percent of the simulations.

Barcelona -- Bayer Leverkusen
(Barcelona leads 3-1)

Barcelona takes an almost insurmountable 3-1 edge back to Nou Camp for the second leg. Barca is unbeaten in its last 13 European home matches, and a win would give the defending champs their fifth straight appearance in the quarterfinals. Leverkusen has not recorded an away victory in the Champions League in a decade, and SPI gives Leverkusen only a 2.3 percent chance to score the three or more goals needed to advance.

Leverkusen has scored three or more goals six times this season but has won by a three-goal margin just twice. Unfortunately for the German side, Barcelona has allowed three or more goals in a match just twice this season in all competitions and is undefeated at home in La Liga this season, conceding five times in 13 league matches. According to SPI, Leverkusen would have needed a three-goal advantage entering this leg in order to be the favorite.