Defending Olympic champion Andy Murray reached the semi-finals of the men's tennis on Friday after winning a third-set tiebreak to scrape through a testing encounter against American Steve Johnson.

Spaniard Rafa Nadal also secured a spot in the semi-finals with a hard fought victory against home court favorite Thomaz Belucci of Brazil, rallying back after losing the first set.

Britain's Murray, the world number two, edged into the last four with a 6-0 4-6 7-6(2) victory but was pushed to the limit by the 22nd-ranked Johnson, who came back to win the second set after being brushed aside in the first.

The players traded service breaks and blazing passing shots in the final set but Murray showed steady nerves to win the tiebreak, closing out the hardcourt match with a deftly placed overhead smash.

"It was a very tough match and I just managed to turn it around in the end," Murray told reporters. "For me it has nothing to do with having won the gold in London. For me it's about trying to win more medals for the country and the team."

Murray will face the winner of a match later on Friday between Japan's Kei Nishikori and Gael Monfils of France.

Nadal, ranked fifth in the world, was tested by No. 54-ranked Belucci in a center court stadium filled with rowdy Brazilian fans, but found his rhythm in the final two sets, winning by 2-6, 6-4, 6-2.

Nadal will take on either Argentina's Juan Martin del Potro or Spain's Roberto Bautista Agut in the semi-finals.

The Spaniard will also compete for a gold medal in the men's doubles final later Friday, where he is teamed with compatriot Marc Lopez against a duo from Romania, Florin Mergea and Horia Tecau.

Puerto rican goes for gold

In the first women's singles semi-final of the Games, Puerto Rico's Monica Puig beat Petra Kvitova of the Czech Republic for a spot in the gold-medal match.

Puig, ranked 34 in the world, won a decisive third set to go through 6-4 1-6 6-3. She will compete in the final for what may be her country's first medal of the Games, and its first ever gold.

"It would be unbelievable and it would be the biggest honour in the world," Puig told reporters about the prospect of winning gold. "The island is full of such bad news all the time, so every time there's a Games and somebody from the island wins a medal everything stops and I know how happy people get."

Island nation Puerto Rico, which has been facing a deep financial crisis, has captured a total of just eight medals at past Games, mostly in boxing.

Puig will take on either American Madison Keys or Angelique Kerber of Germany in the final.

The Olympic tennis tournament has been marked by upsets with both the men's and women's top seeds, Serbia's Novak Djokovic and American Serena Williams, knocked out in the early rounds.

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