Murray, who beat Roger Federal on the grass of Wimbledon to claim the gold medal at London 2012, was never able to dominant his opponent but always had enough at the big moments to edge ahead.

He eventually prevailed 7-5, 4-6, 6-2, 7-5 in a match lasting over four hours to claim his country's fifth gold medal of the day - a record at an Olympics outside London.

"Tonight was one of the hardest matches that I’ve had to play for a big title," the 29-year-old, who carried the British flag at the Opening Ceremony, said.

"I think the US Open final when I played Novak [Djokovic] to win my first slam was very hard but tonight I found it really difficult. 

"Emotionally it was tough, physically it was hard with so many ups and downs in the match.

"The fact that it [winning two Olympic gold medals] has never been done before shows it’s a very difficult thing to do and I’m very proud to have been the first one to have done it."

The world number two blew a 4-1 lead before fighting back to win the first set 7-5 with a trademark backhand passing shot.

Del Potro swiftly responded, however, breaking early in the second before holding on to take the set 6-4.

Murray won four successive games as his opponent tired to win the third set 6-2.

The real drama, though, was still to come.

As blue and white clad fans sung and roared, the Argentinian found a second wind as four successive breaks of serve opened the fourth set.

Both players had also played doubles this week, so errors were always likely to creep in with the fatigue.

But the level of tennis was also sublime, with Del Potro's booming forehand coming against the brick wall of the Murray defence.

The defending champion required all of his trickery and match nous to stay in touch before he broke back for 5-5, held his own serve, and then eventually made the breakthrough.

Both players then embraced warmly at the net, clearly equally exhausted.

It continued an 18 winning streak for Murray which has also seen titles at Queens and a third Grand Slam triumph at Wimbledon.

Japan's Kei Nishikori had earlier won a similarly tough clash with Spain's 14-time Grand Slam winner Rafael Nadal for the bronze medal.

He squandered a 5-2 lead in the second set before winning 6-2, 6-7, 6-3.

There was disappointment for Venus Williams in her pursuit of a fifth Olympic gold when she and partner Ram Rajeev lost an all-American mixed doubles final against Bethanie Mattek-Sands and Jack Sock.

Mattek Sands and Sock recovered from a set down to win 10-7 in a match tie break after the first two sets ended 6-7, 6-1.

Russian pair Elena Vesnina and Ekaterina Makarova secured the first title of the day in the women's doubles, beating Switzerland's Martina Hingis and Timea Bacsinszky 6-4 6-4.