Murray will face Japan`s Kei Nishikori who edged out France`s Gael Monfils 7-6(4) 4-6 7-6(6).
Rio de Janeiro: London Olympic champion Andy Murray reached the semi-finals of the men`s tennis on Friday after winning a third-set tie-break to scrape through a testing encounter against American Steve Johnson.
Spain`s Rafa Nadal also secured a spot in the semi-finals with a hard fought victory against local favourite 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.
The players traded service breaks and blazing passing shots in the final set but Murray showed steady nerves to win the tie-break, closing out the 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 Japan`s Kei Nishikori who edged out France`s Gael Monfils 7-6(4) 4-6 7-6(6) in a tense third set tie-break after facing repeated match points.
Nadal, ranked fifth in the world, was tested by No. 54-ranked Belucci in a centre court stadium filled with passionate Brazilian fans, but found his rhythm in the final two sets, to clinch the 2-6 6-4 6-2 victory.
"The atmosphere was tough, but at the same time I enjoy it," Nadal told reporters. "I managed to come back in some tough situations."
Nadal will take on Argentina`s Juan Martin del Potro, who advanced with a 7-5 7-6(4) win over Spain`s Roberto Bautista Agut, drawing enthusiastic chants from rafters packed with blue-and-white clad fans from neighbouring Argentina.
Nadal will also team up with Marc Lopez in the men`s doubles final against Romania`s Florin Mergea and Horia Tecau.
In the first women`s singles semi-final, Puerto Rico`s Monica Puig beat Petra Kvitova of the Czech Republic to give her the chance to win her country`s first Olympic gold medal.
The 34th-ranked Puig 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 Rio Games.
"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."
Puerto Rico, which has been facing a deep financial crisis, has won just eight medals at past Games, mostly in boxing.
Puig will take on Australian Open champion Angelique Kerber in the final after the German beat Madison Keys 6-3 7-5 in the other semi-final.
(additional reporting by Anthony Boadle; Editing by Clare Fallon/Andrew Hay/Greg Stutchbury)