Cuba's Yordenis Ugas sent Manny Pacquiao spinning to a dramatic upset defeat on Saturday, outclassing the Filipino boxing icon with a composed unanimous points victory at the T-Mobile Arena.

In his first fight after a two-year absence from the ring, Pacquiao was made to look every bit of his 42 years as the younger Ugas dominated with a stinging jab and cleaner punching to retain his WBA welterweight belt.

Ugas, 35, had only been drafted in to face Pacquiao earlier this month after the eight-division world champion's original opponent Errol Spence Jr. suffered an eye injury in training.

But the defending champion seized his opportunity brilliantly to win on all three cards, with one judge scoring it 115-113 and two others 116-112.

"That's boxing," a deflated Pacquiao said afterwards.

"I had a hard time in the ring making adjustments. My legs were tight. I'm sorry I lost tonight, but I did my best."

It was a sweet victory for Ugas, who had been awarded the WBA title after Pacquiao was stripped of the belt by the sanctioning body earlier this year for inactivity.

"I'm very excited but most of all, I want to thank Manny Pacquiao for giving me this opportunity in the ring today," said Ugas, who improves to 27-4 with 12 knockouts. "We only had two weeks of training but I listened to my corner and it all worked out.

"I told you I am the champion of the WBA and I showed it tonight. A lot of respect for (Pacquiao), but I won the fight."

The defeat is almost certain to renew doubts about Pacquiao's future in the sport he has graced through 72 fights spanning 26 years since 1995.

However Pacquiao could have few complaints about the eighth defeat of his glittering career.

Although the Filipino was aggressive throughout, advancing with flurries of punches from the opening round, few of his blows did inflicted damage on Ugas, who cleverly used his advantages in reach and height to frustrate Pacquiao.

Ugas meanwhile scored consistently with crisper punches that often rocked Pacquiao back on his heels.

Ugas's jolting left jab caused problems for Pacquiao throughout, and he repeatedly did damage with a series of powerful rights.

After a cagey opening round, Ugas quickly asserted himself in the middle rounds, startling Pacquiao with a left-right combination in the fifth before ending the sixth with another hard right.

A T-Mobile crowd of 17 438 tried to rally Pacquiao with chants of "Manny, Manny" throughout but the Filipino's legion of admirers grew steadily more subdued as the rounds wore on.

Pacquiao, with a cluster of red marks on his cheek and forehead a testament to Ugas's increasing effectiveness, continued to come forward as he sought a big round to change the complexion of the contest.

But Ugas would not be denied, and he had Pacquiao scrambling onto the defensive once again after landing a double left jab and a right hook in quick succession in the 10th, before closing out the final rounds to win. (AFP)