Manny Pacquiao and Juan Manuel Marquez -- the No. 3 and No. 5 fighters in SI.com's pound-for-pound ratings, respectively -- are fighting for a fourth time on Saturday at the MGM Grand Garden Arena in Las Vegas (9 p.m. ET, HBO PPV, $59.99). It is a non-title fight, with Pacquiao having lost his WBO welterweight championship in controversial fashion to Timothy Bradley in June.

Pacquiao and Marquez -- both surefire Hall of Famers -- fought to a controversial draw in 2004, a narrow split-decision win for Pacquiao in 2008, and a still-more-controversial majority-decision win for Pacquiao in 2011. Yet a strong case can be made that Marquez won all three. While Pacquiao scored four knockdowns in their three meetings, it was Marquez who won the greater number of rounds.

These two have fought 36 rounds with only seven points separating them on the scorecards. The first bout at featherweight saw Pacquiao drop Marquez three times in the first three minutes 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 -- the best fight of the three.

Yet while Marquez enjoyed a stellar career while campaigning at lightweight, it was Pacquiao who blossomed into a crossover superstar and global phenomenon, 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. That was the backdrop for the third fight, which saw Marquez nearly engineer a seismic upset despite the 7-to-1 odds against him.

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 hasn't registered a knockout since 2009, even hearing the rare sound of boos in the later stages of a lopsided points victory over Shane Mosley in May 2011. Both fighters are motivated to give this rivalry an definitive resolution, ideally in the form of a stoppage win.

If the three previous installments have proven anything, it's that Pacquiao and Marquez cancel out one another's strengths.

Pacquiao has shown impressive technical improvement under trainer Freddie 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. Yet at 33, he's shown signs of decline over his past three fights with Mosley, Marquez and Bradley, no longer fighting pell-mell for three minutes of every round. Hall of Fame trainer Nacho Beristain, Marquez's cornerman, says Pacquiao's development into a schooled boxer plays to their advantage -- he's no longer as wildly unpredictable as he once was.

Marquez, 39, is a skilled veteran with a world-class chin and recuperative powers -- he's never been stopped in 61 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 nightmarish style matchup for Pacquiao. Watch as the Mexican systematically drifts backward and to the left, neutralizing Pacquiao's TNT-packed left hand and feint attacks. He knows exactly how to defend against Pacquiao -- where to take risks, when to dial it back -- yet thus far it's yet to pay off with a victory. In the bloody ballet between aggressor and counterpuncher, the judges have tended to favor Pacquiao's come-forward style.

Roach, a Hall of Famer and 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 Beristain, a recent inductee to Canastota. Advantage to Pacquiao, but only by the slimmest of margins.

Marquez opened as a 3-to-1 underdog at the MGM Grand sports book, while Pacquiao was listed as a 1-to-4 favorite.

We've learned so much about these two fighters over three fights spanning seven years, yet questions persist. Pacquiao has always benefited from being the aggressor -- his style is easier to see and easier to score than Marquez's backward-moving counterpunching -- but will it be enough to escape with a third straight close decision if it goes to the cards? Will the desire for a knockout draw either fighter out of their comfort zone? Will one or the other fall off an age cliff? It says here that Pacquiao has slipped just enough for the bulked-up Marquez to stymie, frustrate and counter his way to a legacy-sealing win. Marquez by split decision.

Join the conversation about Pacquiao-Marquez IV 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)

· Yuriorkis Gamboa vs. Michael Farenas, 12 rounds, for vacant WBA interim junior lightweight title

· Miguel Vazquez vs. Mercito Gesta, 12 rounds, for Vazquez's IBF lightweight title

· Javier Fortuna vs. Patrick Hyland, 12 rounds, for vacant WBA interim featherweight title

· Non-PPV bouts: Dodie Boy Penalosa vs. Jesus Lule Raya, 8 rounds, junior lightweights; Ernie Sanchez vs. Coy Evans, 8 rounds, featherweights; Jazzma Hogue vs. Alexis Hernandez, 4 rounds, featherweights; Jose Ramirez vs. Corey Siegwarth, 4 rounds, lightweights

· HBO's Jim Lampley, Larry Merchant and Roy Jones Jr., will be ringside for the main event and undercard.

· The referee assigned to the main event is Kenny Bayless. The judges are Adelaide Byrd (Las Vegas), Steve Weisfeld (New Jersey) and John Keane (England). (Read more about the selection process here.)

MANNIX: Pacquiao and Marquez look to close the book on their rivalry

GRAHAM: Marquez fans make themselves heard at weigh-in

GRAHAM: Everything you need to know about the three people who will probably decide the fight

MANNIX: Bob Arum, one of boxing's most powerful promoters, still hustling hard

MANNIX: What a difference a year makes for Arum, 50 Cent

MANNIX: Marquez, Hernandez deny steroid accusations made by Roach

SI.com's experts' predictions on Pacquiao-Marquez IV

PHOTOS: Pacquiao's Biggest Fights | Pacquiao: Outside the Ring

VIDEO: How Pacquiao and Marquez arrived at fourth fight | Pacquiao music video | Marquez music video | Watch complete first episode of 24/7 | Watch complete second episode of 24/7 | Watch complete third episode of 24/7