LAS VEGAS -- Somewhere, Robert Fitzsimmons is smiling.

The last middleweight champ to defeat a heavyweight title holder is probably smiling the same way Roy Jones was smiling in the final round of the signature performance of his accomplished career.

It's been a while, more than 100 years, in fact, since a natural-born middleweight defeated a top heavyweight. Fitzsimmons did it with a single punch to Jim Corbett's solar plexus that downed the heavyweight champ in the 14th round of their 1897 showdown. Jones did it with a lot of punches. He didn't stop John Ruiz, but he beat him easilyy. Maybe he could have stopped Ruiz if he had two more rounds like Fitzsimmons did.

As it was, Jones won a unanimous decision over 12 rounds of one-sided but intense action. Judge Stanley Christodoulou scored the bout 118-110 for Jones, a near shutout. Judges Duane Ford and Jerry Roth scored the bout 117-111 and 116-112, respectively.

How did he do that? And how did he do it so easily?

"Because I'm Roy Jones," the new WBA heavyweight champ said after the fight. "I'm always going to be Roy Jones. I'm not going to change until someone changes me."

Ruiz was not the fighter to "change" the pound-for-pound best fighter in the world. In fact, Jones was the one who changed Ruiz. The reigning light heavyweight champ reduced the WBA champ into just another hapless opponent.

But how did he do it?

Jones, a former middleweight and super middleweight title holder, won the WBA heavyweight title by fighting like a heavyweight, not a 160 pounder. He stood his ground, he took the few solid shots Ruiz landed, and he easily avoided the other punches.

Jones earned earned Ruiz's respect with his unexpected power. And then his skills and phenomenal speed took over.

After an even opening frame, Jones took away Ruiz's best weapon, the left jab, by the third round. By the fourth round, Jones was in complete control of the bout and Ruiz was falling into the same pattern as many good light heavyweights Jones had fought -- Lou Del Valle, Reggie Johnson, David Telesco -- were forced into. He stood in front of Jones, without punching. He bulled Jones to the ropes and hit the smaller man's arms before eating hard uppercuts and absorbing accurate body shots.

Ruiz stalked and stalked and stalked and wondered, "What the heck am I going to do with this guy?"

In the second and third rounds, Jones landed crisp hooks while backing up and scored with sharp rights while stepping in. Jones landed an uppercut in the fourth that splattered Ruiz's nose, covering the heavyweight's face in blood.

By the middle rounds, as his confidence began to soar, Jones reduced Ruiz to one of those helpless middleweights he used to toy with in the early to mid-1990s. Ruiz was no longer the WBA champ: Jones turned him into Vinny Pazienza; he turned Ruiz into Tony Thorton.

Well, maybe that's not fair.

Jones couldn't stop Ruiz, but he could contain and control the New England heavyweight. Let's say that Jones turned Ruiz into James Toney. Ruiz was out-classed and even out-gunned, but he never stopped trying.

In the fifth round, Jones landed a good uppercut only to get caught with an uppercut from Ruiz. Jones then dared Ruiz to hit him again, driving the crowd into a frenzy.

In the sixth round, Jones landed more hooks, but Ruiz closed the round well, forcing Jones to the ropes. But Jones remained in control, even if though he looked a little gassed in the eighth and ninth rounds.

Jones got his second wind in the 10th and 11th, electing to stand and trade in spots. He took the 12th off, losing the round on all three official scorecards, and still got the packed crowd at the Thomas and Mack Center to chant his name.

The last light heavyweight champ to beat a heavyweight champ was Michael Spinks, who out-pointed Larry Holmes in a close, controversial decision that took place in 1985.

Ruiz is no Holmes, but Jones is just as great as Spinks and Holmes. He proved it Saturday.