Welterweight titleholder Manny Pacquiao, widely considered the world's best pound-for-pound boxer, is fighting recognized lightweight champion Juan Manuel Marquez on Saturday at the MGM Grand Garden Arena in Las Vegas (9 p.m. ET, HBO PPV, $54.99). While Pacquiao's 147-pound title is at stake, the bout is taking place at a catchweight of 144 pounds.

It marks the third meeting between the two future Hall of Famers after a controversial draw in 2004 and a narrow split-decision victory for Pacquiao in 2008, though a strong case can be made that Marquez won both fights. While Pacquiao scored four knockdowns in their two meetings, it was Marquez who won the greater number of rounds.

The first fighter to capture world titles in eight different weight classes (from 112 to 154), Pacquiao ranks No. 1 in SI.com's pound-for-pound ratings. Marquez, a champion at featherweight, super featherweight and lightweight, ranks No. 5.

The two previous meetings between Pacquiao and Marquez were just about as close as you can get: Pacquiao holds a 679-678 points lead if you add up the scorecards from both fights. The first bout at featherweight saw Pacquiao drop Marquez three times in the opening round only to find himself outboxed by the Mexican's shrewd counterpunching over the last 11 rounds, while Pacquiao did just enough to eke out a one-point victory in the rematch four years later at super featherweight.

But while Marquez has enjoyed a stellar career while campaigning at lightweight, it's Pacquiao who's blossomed into a crossover superstar and global phenomenon in three years since they last met, adding world titles at 135, 140, 147 and 154 pounds, winning election to Congress in the Philippines and being named one of Time magazine's 100 most influential people.

For a fighter who's made his name on exciting, crowd-pleasing performances, Pacquiao's most recent outings have left fans with a tinge of disappointment. He's gone the distance in three consecutive fights, even hearing the rare sound of boos in the later stages of a lopsided points victory over Shane Mosley in May. The motivation to deliver an emphatic stoppage victory to cap the trilogy -- stoked by Marquez's choice to wear a T-shirt that read MARQUEZ BEAT PACQUIAO TWICE!! during the press tour in the Philippines -- gave Pacquiao a sharpened focus in training camp that left an impression on everyone he came in touch with. "I've never seen him work so hard for a fight," trainer Freddie Roach said last week. "His work ethic is great in every fight but for this one he has a little extra spark."

Despite the razor-thin difference between the fighters in their previous meetings, there are a number of compelling reasons Pacquiao is an overwhelming favorite to extend his current streak of 14 straight victories. There's age: the 32-year-old Pacquiao is far closer to his athletic peak than Marquez, who is 38. There is also the natural size difference: Pacquiao has blossomed into a legitimate welterweight (with victories over bona fide 147-pounders Miguel Cotto, Josh Clottey, Antonio Margarito), while Marquez is a bulked-up lightweight whose lone previous foray above the 140-pound threshold was a woefully one-sided loss to Floyd Mayweather.

Most important perhaps is Pacquiao's technical improvement under Roach. Where his right hand was once a mere throat clearing for the punishing left, Pacquiao is now essentially an ambidextrous fighter, a versatile boxer-puncher whose outrageous punch volume is enough to keep opponents on their back foot for rounds at a time.

Marquez is a skilled veteran with a world-class chin and recuperative powers -- he's never been stopped in 59 pro fights -- whose exquisite technique and textbook jabs, hooks and combinations make him a true fight fan's fighter. And if the first two fights proved anything, it's that Marquez's well-timed counterpunching is a bad style matchup for Pacquiao. Still, none of it is enough to convince the oddsmakers that Marquez has anything but a puncher's chance. Remember: Marquez was at a speed disadvantage even before he added all that muscle.

Roach, a five-time Trainer of the Year, is one of the game's best corner men whose teaming with Pacquiao is becoming one of boxing's historic fighter-trainer partnerships. Marquez is a longtime student of recent Hall of Fame inductee Nacho Beristain. Advantage to Pacquiao, but only by the slimmest of margins.

Oddsmaker William Hill lists Marquez as a 6-to-1 underdog, while Pacquiao is a 9-to-1 favorite.

This is a mismatch. A saleable one given how close the first two fights were, but those happened at 126 and 130 pounds. The contracted weight of 144 for Saturday's rubber match will prove a bridge too far for Marquez, not unlike the wire-to-wire beatdown he absorbed at the hands of Floyd Mayweather in 2009. The way Marquez has bulked up pretty much lays his strategy bare: at 38, he knows winning a decision against an opponent as well-conditioned as Pacquiao is unlikely, so his best chance is to trade bombs with the champ in the hope of catching him with a big shot. In the end Manny's power, hand speed and accuracy will prove too much for the Mexican warrior. The good news: it will be fun while it lasts. Pacquiao by fifth-round TKO.

Join the conversation about Pacquiao-Marquez III on Twitter. Track the hashtag #PacMarquez to see who's tweeting what about Saturday's fight.

· @TRBoxing (Top Rank Boxing)

· @HBOboxing (HBO Boxing)

· @ChrisMannixSI (SI.com's Chris Mannix)

· @BryanAGraham (SI.com's Bryan Armen Graham)

· @timdalhberg (Tim Dahlberg, Associated Press boxing columnist)

· @orangerose (Gary Andrew Poole, Time contributing writer and Pacquiao's biographer)

· @GarethBOXUFC (Gareth A. Davies, Daily Telegraph boxing columnist)

· @NYPost_Willis (George Willis, New York Post boxing columnist)

· @latimespugmire (Lance Pugmire, Los Angeles Times boxing columnist)

· @kieranmulvaney (Kieran Mulvaney, Reuters and ESPN boxing columnist)

· @danrafaelespn (Dan Rafael, ESPN boxing writer)

· @AmyZopfi (Amy Zopfi, Event Services Manager at MGM Grand Garden Arena)

· @mgmgrand (MGM Grand Hotel)

· Timothy Bradley Jr. vs. Joel Casamayor, 12 rounds, for Bradley's WBO junior welterweight title

· Mike Alvarado vs. Breidis Prescott, 10 rounds, junior welterweights

· Luis Cruz vs. Juan Carlos Burgos, 12 rounds, junior lightweights

· Non-PPV bouts: Dennis Laurente vs. Ayi Bruce, 8 rounds, welterweights; Fernando Lumacad vs. Joseph Rios, 8 rounds, junior bantamweights; Jose Benavidez Jr. vs. Sammy Santana, 6 rounds, junior welterweights; Victor Pasillas vs. Jose Garcia, 4 rounds, featherweights

· HBO's Jim Lampley, Emanuel Steward, Max Kellerman and Harold Lederman will be ringside for the main event and undercard.

· The referee assigned to the main event is Tony Weeks, whose previous high-profile assignments include Pacquiao-De La Hoya, Mayweather-Marquez, Kelly Pavlik-Jermain Taylor II, Marcos Maidana-Erik Morales and Diego Corrales-Jose Luis Castillo I. The judges are Dave Moretti, Robert Hoyle and Glenn Trowbridge.

· For the first time, HBO is telecasting the weigh-ins. Fans can watch Friday at 6 p.m. ET/3 p.m. PT on HBO or online at http://www.insidehboboxing.com/.

· HBO will air six hours of fight-related programming on multiplex channel HBO Zone on Saturday leading up to the fight (from noon to 6 p.m. ET/PT). Jim Lampley will host the HBO Zone Roadblock, which includes a one-hour Fight Day Now! show from the MGM Grand in Las Vegas at 4 p.m. that will give fans all the latest news as the main event approaches.

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VIDEO: Pacquiao-Marquez Under the Lights | Pacquiao-Marquez III: Fight Preview | Pacquiao's grand entrance to Vegas | Master trainers guide Pacquiao, Marquez | Watch complete first episode of 24/7 | Look Ahead to Pacquiao-Marquez III | Manny Pacquiao: Ring Life | Juan Manuel Marquez: Ring Life