Timothy Bradley stops Brandon Rios in the ninth round of their WBO welterweight title bout Saturday in Las Vegas.

John Locher AP

LAS VEGAS -- Timothy Bradley won a fight he needed badly Saturday night, stopping Brandon Rios in the ninth round to stake his claim for a title in a welterweight division that is wide open following Floyd Mayweather Jr.'s retirement.

Bradley (33-1-1) dominated from the opening bell against a sluggish Rios (33-3-1), using his speed and superior boxing skills to win round after round before suddenly dropping Rios with a left hook to the body in the ninth round. Rios got up at the count of eight but offered no resistance to Bradley when the fight resumed and quickly went down under another flurry of punches at 2:49 of the round.

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Bradley retained his piece of the 147-pound title with an impressive performance under new trainer Teddy Atlas. He won every round on all three ringside scorecards but hadn't seemed to seriously hurt Rios before the fight came to a sudden end.

Perhaps more importantly, Bradley served notice that he's a name to be reckoned with in the 147-pound division. He might have even been impressive enough to get a third fight against Manny Pacquiao sometime next spring in what would be Pacquiao's last fight.

''That's the best Bradley I've ever seen,'' said Bob Arum, who promotes both fighters.

Bradley brought in Atlas, more known recently as a ringside announcer for ESPN, to sharpen his skills and stop him from getting hit too much.

It worked to perfection in a fight that Bradley controlled from the first punch.

''I did exactly what Teddy told me to do,'' Bradley said. ''I took a piece of him every round.''

Bradley set the pace for the fight right away, landing a series of head shots early in the first round. Rios, who ballooned up in weight after Friday's weigh-in, plodded forward trying to land a big punch but was never able to land more than one shot at a time.

Rios said he would probably retire after the loss.

''The better man won. I have no excuses,'' Rios said. ''I think I'm done.''

Rios, who had to weigh in twice the day before the fight after coming in slightly over the 147-pound limit, weighed in unofficially at 170 pounds on the HBO scale in his dressing room. Bradley, who weighed 146 at the official weigh-in, was 155 in his dressing room.

There was a lot at stake for both fighters in a crossroads bout for each. Bradley, who was 1-1-1 in his past three fights, was trying to stay relevant as a top level welterweight, while Rios was trying to prove that he was a better fighter than the one who lost badly to Pacquiao in 2013 before rebounding to win his past two fights.

But it was Bradley who had the big night, throwing 570 punches and landing nearly half of them (254). Rios was ineffective in landing just 81 of 454 punches.

''The game plan was to take pieces and break him down little by little,'' Atlas said. ''Don't get greedy and then by like a piranha. Take pieces every round and then finish him off.''

In the semi-main event, two-time Olympic gold medalist Vasyl Lomachenko defended his piece of the featherweight title by stopping Mexico's Romulo Koasicha with a series of body shots in the 10th round.

Lomachenko dominated the fight most of the way, peppering Koasicha with lefts and rights though he was unable to put him down. The Ukranian finally did in the 10th round, though, landing three lefts to the body that dropped Koasicha to one knee against the ropes.

Koasicha did not try to get up as referee Robert Byrd counted him out at 2:35 of the 10th round.

''If I wanted to knock him out I would have done it earlier,'' Lomachenko said. ''I was having fun in there.''

Lomachenko won every round on all three scorecards, and his dominance was reflected in stats that showed him landing 334 punches to just 75 for Koasicha.

Koasicha, who was stopped for the first time as a pro, fell to 25-5.