For the first time in more than 18 months an undisputed UFC heavyweight champion fight will go down as Cain Velasquez (13-1 MMA, 11-1 UFC) and Fabricio Werdum (19-5-1 MMA, 7-2 UFC) are set to unify the gold in UFC 188’s main event.

Velasquez has not defended his title since October 2013 due to injury, and in the meantime Werdum captured the interim belt. The two titleholders will now meet to unify the belts and decide who has the rightful claim to being called the No. 1 heavyweight in the world.

The matchup was originally meant to take place this past November as part of the UFC’s debut event in Mexico. It didn’t happen due to a Velasquez injury, but now that everyone is healthy, the octagon will once again touch down at Mexico City’s Mexico City Arena.

For more on the numbers behind the sixth title unification bout in UFC history (third in the heavyweight division), as well as the other 11 matchups slated for the card, check out 60 pre-fight facts about UFC 188.

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Main event

Velasquez returns to competition for the first time since his fifth-round TKO of Junior Dos Santos at UFC 166 in October 2013. His 602-day layoff is the longest of his career dating back to his October 2006 professional debut.

Velasquez’s two consecutive UFC heavyweight title defenses are tied with Brock Lesnar, Tim Sylvia and Randy Couture for the most of any champion in divisional history.

Velasquez is one of only five fighters in UFC history to win multiple championships in a single weight class. Couture, Sylvia, Georges St-Pierre and Matt Hughes also accomplished the feat.

Velasquez’s four-fight UFC winning streak in heavyweight competition is the third longest active streak in the division behind Andrei Arlovski (six) and Werdum (five).

Velasquez’s 11 victories in UFC heavyweight competition are tied for the third most in divisional history behind Frank Mir (15) and Arlovski (13).

Velasquez’s nine stoppage victories in UFC heavyweight competition are the fourth most in divisional history behind Mir (12), Arlovski (11) and Gabriel Gonzaga (11).

Velasquez’s nine knockout victories in UFC heavyweight competition are tied with Arlovski for the most in divisional history.

Velasquez’s nine knockdowns landed in UFC heavyweight competition are the third most in divisional history behind Dos Santos (12) and Arlovski (10).

Velasquez is the only fighter in UFC history to land 100 or more significant strikes and 10 or more takedowns in the same fight.

Velasquez is one of two fighters in UFC history to land 200 or more significant strikes in three separate UFC bouts. St-Pierre is the other to accomplish the feat.

Velasquez has landed 1,257 total strikes in his UFC career, the most of any heavyweight in history.

Velasquez’s striking differential rate of +4.52 in UFC heavyweight competition is the highest rate in divisional and overall UFC history.

Velasquez has completed 29 takedowns in UFC heavyweight competition, the most in divisional history.

Velasquez lands 6.21 significant strikes per minute of fighting in UFC heavyweight competition, the highest rate in divisional history and second highest rate overall in UFC history behind Jessica Andrade (6.76).

Velasquez’s knockout of Dos Santos at 3:09 of Round 5 at UFC 166 stands as the latest stoppage ever in UFC heavyweight bout.

Werdum, 37, is the oldest of the 24 fighters scheduled to compete at the event.

Werdum is one of nine fighters in UFC history to win an interim championship belt. Four of the previous eight went on to be crowned the undisputed champion.

Werdum’s five-fight UFC winning streak in heavyweight competition is the second longest active streak in the division behind Arlovski (six).

Werdum’s five-fight winning streak in MMA competition is the longest of his pro career.

Werdum is the only fighter in MMA history to submit both Antonio Rodrigo Nogueira and Fedor Emelianenko. Dan Henderson is the only man aside from Werdum to hold wins over both Nogueira and Emelianenko.

Werdum has landed 57.4 percent of his significant strike attempts in UFC heavyweight competition, the third highest accuracy among active heavyweights behind Alistair Overeem (74.5 percent) and Velasquez (58.1 percent) and fifth highest in divisional history.

Werdum has committed to 21 submission attempts in his past 23 fights. He’s also landed 28 takedowns in the same amount of time (since 2003). He’s 12-3 in fights in which he lands at least one takedown.

Werdum’s only defeats in the past 10 years are to fighters who once held the UFC, PRIDE or Strikeforce heavyweight belts.

Werdum has suffered four of his five career losses by decision.

Co-main event

Gilbert Melendez (22-4 MMA, 1-2 UFC) will enter the contest after the first stoppage loss of his career. He suffered a second-round submission loss to Anthony Pettis at UFC 181.

Melendez reigned as Strikeforce champion for 1,135 days, the longest title run of any lightweight champion in UFC/WEC/Strikeforce combined history.

Melendez’s 11 victories under the now-defunct Strikeforce banner stand as the most in company history.

Melendez’s 749 significant strikes landed during his Strikeforce career were the most in the promotion’s history.

Eddie Alvarez (25-4 MMA, 0-1 UFC), a former Bellator MMA lightweight champion and USA TODAY Sports/MMAjunkie’s No. 8 ranked 155-pound fighter, makes his second UFC appearance after losing his octagon debut at UFC 178 in September.

Alvarez suffered the first decision loss of his career against Donald Cerrone at UFC 178. He’d previously emerged victorious every time a fight went the distance.

Alvarez has earned 21 of his 25 career victories by stoppage.

Remaining main card

Kelvin Gastelum’s (10-1 MMA, 5-1 UFC) will return to the UFC middleweight division after a five-fight stint at welterweight in which he missed weight twice.

Nate Marquardt (33-14-2 MMA, 11-7 UFC) fell to 1-1 since he returned to the UFC middleweight division in June 2014.

Marquardt will enter the event with just one victory in his past five bouts.

Marquardt will compete in his 17th UFC middleweight bout, tied for the fourth most appearances in divisional history behind Chris Leben (22), Yushin Okami (18) and Michael Bisping (18).

Marquardt’s 11 victories in UFC middleweight competition are the fifth most in divisional history behind Anderson Silva (13), Okami (13), Bisping (12) and Leben (12).

Marquardt’s seven stoppage victories in UFC middleweight competition are third most in divisional history behind Silva (11) and Leben (nine).

Marquardt’s nine knockdowns landed in UFC middleweight competition are the second most in divisional history behind Silva (12).

Marquardt’s four knockdowns landed against Dean Lister at UFC Fight Night 8 stand as the most ever in a UFC middleweight fight. His four knockdown performance is also tied for second largest in UFC history, behind only Forrest Petz’s five knockdowns of Sammy Morgan at UFC Fight Night 6.

Marquardt’s 21-second knockout of Demian Maia at UFC 102 stands as the fifth fastest knockout in UFC middleweight history.

Marquardt lands 63 percent of his takedowns in UFC middleweight competition, the highest rate in divisional history.

Marquardt has attempted 14 submissions in UFC middleweight competition, tied for the third most in divisional history behind Kendall Grove (17) and Thales Leites (15).

Charles Rosa (10-1 MMA, 1-1 UFC) has earned all of his career victories by stoppage.

Rosa’s submission of Sean Soriano at 4:43 of Round 3 at UFC Fight Night 59 marked the latest submission ever in a three-round UFC featherweight contest.

Rosa put on a grappling exhibition in his last fight when he scored five takedowns, completed 10 guard passes and attempted four submissions.

Tecia Torres (5-0 MMA, 1-0 UFC) has earned all five of her career victories by decision.

Torres is the only fighter to own a victory over UFC women’s strawweight Paige VanZant.

Torres outlanded Angela Magana 96-to-37 in significant strikes at The Ultimate Fighter 20 Finale. Her +59 advantage is the second largest margin in a UFC women’s strawweight fight.

Angela Hill (2-0 MMA, 1-0 UFC) has the fewest pro fights of any active female member of the UFC roster.

Hill registered the first knockdown in the history of the UFC women’s strawweight division when she dropped Emily Kagan at the TUF 20 Finale.

Hill outlanded Emily Kagan 107-to-41 in significant strikes at The Ultimate Fighter 20 Finale. Her +66 advantage is largest margin in a UFC women’s strawweight fight.

Preliminary card

Henry Cejudo (8-0 MMA, 2-0 UFC) is one of three Olympic gold medalists fight in the UFC, along with Kevin Jackson and Mark Schultz. He accomplished the feat in freestyle wrestling at the 2008 Summer Olympics.

Cejudo is the only Olympic gold medalist to fight in the UFC since the organization was purchased by parent company Zuffa.

Cejudo has outlanded his UFC opponents 122 to 47 in his two UFC appearance while landed with 51 percent striking accuracy.

Chico Camus (15-5 MMA, 4-2 UFC) was successful in his UFC flyweight debut when he defeated Brad Pickett at UFC Fight Night 57.

Camus has earned his past seven victories by decision. That includes all four of his wins under the UFC banner.

Johnny Case (20-4 MMA, 2-0 UFC) has earned 18 of his 20 career victories by stoppage.

Case enters the event on a career-high 10-fight winning streak. He hasn’t suffered a defeat since October 2010.

Francisco Trevino (12-0 MMA, 1-0 UFC) returns to competition for the first time since UFC 171 in March 2014. His 455-day layoff is the longest of his career dating back to his August 2008 pro debut.

Augusto Montano (14-1 MMA, 1-0 UFC) has earned all of his career victories by stoppage. He has finished 12 of those opponents inside the first round.

Cathal Pendred (16-2-1 MMA, 3-0 UFC) enters the event on a career-high 11-fight undefeated streak. He hasn’t suffered a defeat since December 2010.

Pendred has earned seven of his past nine victories by decision. He’s never lost a fight that’s gone the distance.

Clay Collard (14-5 MMA, 1-1 UFC), 22, is the youngest of the 24 fighters scheduled to compete at the event.

For more on UFC 188, check out the UFC Rumors section of the site.

FightMetric research analyst and live statistics producer Michael Carroll contributed to this story. Follow him on Twitter @MJCflipdascript.