Former five-division titlist Floyd Mayweather Jr. is fighting WBA welterweight champion Shane Mosley on Saturday at the MGM Grand Garden Arena in Las Vegas (9 p.m. ET, HBO PPV, $54.95).

The welterweight bout is a showdown between two of the world's three best fighters regardless of weight. Mayweather ranks either first or second on most experts' pound-for-pound lists (either behind or ahead of Manny Pacquiao), while Mosley ranks no worse than No. 3.

Though Mosley holds the WBA's 147-pound championship by virtue of his knockout of the favored Antonio Margarito in January 2009, Saturday's meeting is a non-title fight. (Mayweather reportedly didn't want to pay the six-figure sanctioning fee.) Mosley will retain the WBA title with a victory, or vacate the strap with a loss.

In addition to HBO's Pay-Per-View telecast, the fight is being shown at more than 250 movie theaters across the United States.

Mayweather, 33, turned pro in October 1996 after a decorated amateur career that culminated with a bronze medal at the Atlanta Olympics. One of the great defensive technicians of all time, "Pretty Boy" has collected titles at super featherweight, lightweight, junior welterweight, welterweight and junior middleweight, cementing his place among the 50 greatest fighters of all time. Critics say Mayweather's defensive, risk-averse style makes for boring fights, but you can't argue with the results -- 40 wins in 40 fights with 25 ending inside the distance -- and the record-breaking pay-per-view numbers when he's the attraction. Floyd is back in action Saturday for the first time since a lopsided victory by unanimous decision over Juan Manuel Marquez in September.

Mosley, who turned 38 in September, is a three-division champion best known for victories over Oscar De La Hoya (twice), Fernando Vargas (twice), Ricardo Mayorga and Margarito. A surefire Hall of Famer on the back end of a 16-year career, "Sugar Shane" has spent most of the past decade in the shadows of better-marketed fighters like De La Hoya. He's tried for some time to get Mayweather in the ring; cynics argue Saturday's fight should have been made years ago.

Three years ago, Mosley admitted in sworn grand jury testimony to using the BALCO designer steroids "the clear" and "the cream" before his second victory over De La Hoya in 2003, but maintains he was unaware the performance enhancers were illegal. Both fighters consented to an unprecedented Olympic-style random blood and urine testing regimen for Saturday's fight under the stringent policy of the U.S. Anti-Doping Agency.

A full-bodied Hall of Fame welterweight, Mosley is thought to be the toughest opponent of Mayweather's career. The veteran boxer-puncher still ranks among the world's best fighters on merit: fluid offensive style, blinding hand speed, experience on the big stage. Problem is, Mayweather matches most of those strengths and trumps whatever's left. It's difficult to pick against Floyd in any fight that can be made today, not just because he's undefeated, but because he's never been in trouble in any of his pro outings. When Mayweather tucks his chin behind his shoulder, he becomes difficult (if not impossible) to hit flush.

Styles make fights. Mosley looked brilliant in his last fight, against Margarito 15 months ago, but the forward-moving Mexican was tailor-made for Mosley, who's at his best when opponents rush in. Against a counterpuncher like Mayweather, he'll have to be the aggressor and that's not his style. Mosley has also been vulnerable through the years against opponents with strong, purposeful jabs -- one of Mayweather's most reliable weapons.

Any questions about Mayweather -- and there aren't many -- center on an alleged lack of power at welterweight. Four-time Trainer of the Year Freddie Roach has repeatedly said Mayweather may be a Hall of Fame fighter at 135 and 140 pounds, but he's been merely pedestrian at 147 and can only dominate against smaller or less skilled opponents. Mosley will attempt to make it a firefight from the get-go and put Floyd's power to the test -- look for a committed body attack -- but forcing Mayweather to stand and trade bombs is easier said than done.

The other prominent question mark is Mosley's long stretch of inactivity. The earthquake in Haiti prompted the cancelation of a scheduled Jan. 30 fight with Andre Berto, who lost eight family members to the tragedy. Will Mosley be sharp enough for Mayweather after more than a year on the shelf? If Miguel Cotto outboxed a 36-year-old Mosley, it's reasonable to assume Mayweather won't be overwhelmed by the 38-year-old vintage.

Intangibly, there have always been questions about Mayweather's focus, but they've never manifested themselves in lackluster performances on fight night.

Oddsmaker William Hill lists Mayweather as a 1-to-5 favorite, with Mosley as a 10-to-3 underdog.

Look for Mayweather to use distance to counter and land between Mosley's combinations in the early rounds, when the judges are most impressionable. There's no question that Mosley will take chances to take Mayweather out -- what does he have to lose? -- but whether he's got enough juice as a 38-year-old (and after a 15-month layoff) to sustain the pressure for a 12-round fight remains to be seen. You can be certain Mosley won't let Mayweather dance his way to a decision like the outclassed Marquez, but Floyd's superior hand speed, ring generalship and legs should be enough to secure a non-controversial verdict. Mayweather by unanimous decision.

Join the conversation about Mayweather-Mosley on Twitter.

· @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)

· @MichaelDavSmith (Michael David Smith, FanHouse boxing writer)

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

· The fourth and final episode of 24/7 MAYWEATHER/MOSLEY debuts Friday (8:30 p.m. ET/PT, HBO). If you missed the first three installments, they're airing back-to-back starting at 7. The series is also available on HBO On Demand.

· DirecTV subscribers can watch the official weigh-in Friday (6 p.m. ET, The 101 Network). The channel is also showing replays of past Mayweather and Mosley fights throughout fight week. Non-subscribers can also catch the weigh-in online at HBO.com.

INSIDE REPORT: Mannix breaks down Mayweather-Mosley from Vegas

VIDEO: Final bell | Fight preview | A Fateful Encounter | The Game Plan

PHOTOS: Mayweather Through the Years | Mosley Through the Years

MANNIX: Stakes couldn't be higher for Mosley against Mayweather

AP: Stiff test for Floyd | Fight sparks Mosley's career | Money still strong draw

REACT: Money Mayweather or Sugar Shane? Weigh in with your prediction