A previously unknown underdog promoted by Marcos Maidana ended one of the longest championship reigns in boxing Saturday night.

Argentina’s Fernando Martinez upset the Philippines’ Jerwin Ancajas by unanimous decision in a brutal, entertaining 12-round, 115-pound championship match that opened Showtime’s tripleheader from The Chelsea inside The Cosmopolitan of Las Vegas. Judges Max De Luca (117-111), David Sutherland (118-110) and Steve Weisfeld (118-110) favored a determined Martinez’s aggression, activity and accuracy and rewarded him with by far the most noteworthy win of his career.

The 30-year-old Martinez (14-0, 8 KOs) won the IBF junior bantamweight championship that Ancajas had owned since September 2016. Ancajas (33-2-2, 22 KOs), who was a 5-1 favorite according to Casears Sportsbook, had made nine successful defenses of his IBF belt before Martinez beat him.

Ancajas, also 30, has a rematch clause in his contract for this optional title defense against the 11th-ranked Martinez. He told Showtime’s Jim Gray in his post-fight interview that he will make use of that contractual right.

The former champion had hoped to face Japan’s Kazuto Ioka, the WBO junior bantamweight champion, in a title unification fight during the summer.

According to Showtime’s punch stats, Martinez landed 427 of an eye-catching 1,046 punches. Ancajas connected on 192 of 816 punches.

Ancajas seemingly needed a knockout entering the 12th round to retain his title, but he couldn’t hurt Martinez, who continued to land flush punches to Ancajas’ head and body.

A determined Martinez unloaded rights and lefts to Ancajas’ head to start what amounted to a largely one-sided 11th round. Ancajas attempted to keep up, but Martinez was the more active, accurate fighter throughout that three-minute period.

Referee Jack Reiss came to Ancajas’ corner before the start of the 10th round and expressed concern for how many punches the champion had taken during the first nine rounds.

Martinez connected with a left-right-left-right combination that backed up Ancajas with just under 50 seconds to go in the 10th round.

Martinez shoved Ancajas to the canvas with just under 1:50 on the clock in the ninth round. They traded flush punches to the head and body during the middle minute of the ninth round.

A left-right-left combination by Martinez made Ancajas stop punching with approximately 45 seconds to go in the ninth round. Martinez then clipped Ancajas with two left hooks that connected with just under 20 seconds remaining in the ninth round.

A right-left combination by Ancajas caught Martinez with just under 15 seconds to go in the eighth round. Martinez made Ancajas work throughout that round and landed various punches to his head and body.

Ancajas’ right hook to the body landed with 1:45 to go in the seventh round. Martinez came forward for most of the seventh round, though, and caught Martinez with a left-right-left combination with approximately 10 seconds on the clock in the seventh round.

Ancajas and Martinez kept up the hectic pace in a back-and-forth sixth round.

Ancajas stood his ground in that round and took Martinez’s best shots until the final minute of that round. Martinez caught Ancajas with a right uppercut and then a left uppercut with just under 50 seconds to go in that sixth round.

Martinez’s left hook up top made Ancajas reset his feet barely 40 seconds into the fifth round. Ancajas and Martinez traded hard head and body shots throughout the second half of an action-packed fifth round.

Ancajas clipped Martinez with a right-left combination just before the midway mark of the fourth round. Ancajas then dug a right hook into Martinez’s body with just over a minute on the clock in the fourth round.

Earlier in the fourth round, an accidental clash of heads appeared to cause the cut over his left eye.

Martinez’s right landed with just over two minutes remaining in the third round. A left and a right by Martinez landed less than 10 seconds later in what was becoming a successful round three for the challenger.

A straight left by Ancajas stopped Martinez in his tracks with just under 50 seconds to go in the third round. Another straight left by Ancajas knocked Martinez off balance toward the end of the third round.

A straight right by Martinez backed up Ancajas 50 seconds into the second round. Martinez landed two more right hands in succession with just over 1:40 on the clock in the second round.

With just under 20 seconds to go in the second round, Ancajas nailed Martinez with a left to the body.

Martinez applied pressure throughout the first round and kept Ancajas on his back foot.

The challenger landed back-to-back left hooks a little less than 40 seconds into their fight. Ancajas caught Martinez with a counter right hook with just under a minute on to go in the opening round.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.