By Joshua Schneyer

RIO DE JANEIRO (Reuters) - Britain's Andy Murray set up a meeting with Argentina's Juan Martin del Potro in the Olympics men's tennis final on Sunday, as he aims to become the first player to retain the singles title.

Meanwhile, Monica Puig clinched the women's singles gold medal for Puerto Rico, defeating Germany's Angelique Kerber. With the victory, the U.S. territory won its first ever Olympic gold medal, and its ninth medal in history.

Del Potro, who entered the Olympic tournament unseeded but gained a huge boost of confidence by defeating world No. 1 Novak Djokovic in the first round, earned his spot in the finals by narrowly defeating Spain's Rafael Nadal.

Ranked 141st in the world after two years plagued by injury, Del Potro came back from a set down against Nadal to win the day, prevailing in a tense tie-break in the decisive third set.

The 27-year-old Argentine, who stands 6 feet 6 inches (1.98m), deployed his barrelling serve and crushing forehand to outmatch Nadal 5-7 6-4 7-6(5).

If he can muster the same kind of power against Murray, the Olympic final is likely to be a thriller.

Murray, 29, defeated Roger Federer of Switzerland to win the gold medal in men's singles in London, where Del Potro was the bronze medalist.

Asked about the prospect of winning a second straight Olympic singles gold, Murray said: "It would mean a lot. It’s obviously not an easy thing to do – that’s why it has not been done before."

Murray's victory on Saturday was far easier for him. He defeated Japan's Kei Nishikori in a commanding display.

The Wimbledon champion and world No. 2 broke Nishikori's serve early on, breezing through the first set 6-1.

Murray served aggressively and quickly closed out the match 6-1 6-4, never giving Nishikori a single chance to break back.

"I think I played really well," Murray told reporters after the match. "I didn’t give him any opportunities on my serves and I was very aggressive when I was returning."

BLUE-AND-WHITE FANS

In the battle between Del Potro and Nadal, the players traded long, bruising rallies to split the first two sets, and each broke the other's serve in the final set. Nadal came up short in the tie-break.

Del Potro has created trouble for the Spanish star before, beating him on his way to a victory at the U.S. Open in 2009, but the Argentine's performance has been inconsistent since that year, and he has been hampered by injury for most of the last two years.

"It means something very big for my career," Del Potro told reporters. "I didn't expect to reach the final beating Djokovic and Rafa."

Del Potro seemed to draw energy from a blue-and-white-clad contingent of fans from Argentina, whose loud chanting from the rafters - typical of Argentine soccer matches - at times drew rebukes from the chair umpire on Saturday.

He entreated the crowd to "treat both players fairly," and at one point reminded them that "this is a tennis match."

Nadal has already claimed a gold medal in Rio with a doubles victory on Friday with partner Marc Lopez. The Spaniard has shown tremendous grit throughout the Rio tournament after months without training. Prior to the Games, Nadal was sidelined with a wrist injury that forced him to sit out Wimbledon this year.

"I just congratulate Del Potro for a great, great event for him," Nadal said after the loss. "I just tried my best until the end, and I'm very satisfied."

PUERTO RICO'S PUIG WINS TERRITORY'S FIRST GOLD

In the Olympic women's singles final, Puerto Rico's Puig defeated Kerber in three sets, 6-4, 4-6, 6-1, to win the island's first gold medal. With the win, Puig also became the first woman ever to win a medal for Puerto Rico.

Twenty-two-year-old Puig, ranked No. 34 in the world, went into the match as the underdog against Kerber, this year's Australian Open champion.

To reach the final, Puig had already defeated two-time Wimbledon champion Petra Kvitova in the semi-finals along the way.

Puig put on an impressive performance on Rio's center court, amid cheers of "Si, se puede" ("Yes, you can") from fans waving Puerto Rican flags. She was able to hit ground stroke winners from all over the court, often painting the lines, and leaving Kerber, the world's No. 2 female player, struggling to effectively deploy her signature, punchy counter-attacking style of play.

Also on Saturday, Kvitova of Czech Republic defeated Madison Keys of the United States to win the bronze medal in the women's draw in three sets, 7-5 2-6 6-2.

American Venus Williams also secured a spot in the mixed doubles final scheduled for Sunday. Win or lose, Williams will become the second tennis player in history to claim five medals at the Olympics.

(Additional reporting by Drazen Jorgic.; Editing by Alison Williams and Bill Rigby)