Roger Federer has defeated John Isner in two tight sets to reach the last four of the men's Olympic singles at Wimbledon.

The Swiss great broke the American's big serve at a crucial stage of the first set before holding his own serve for a 6-4 success.

The second was again a tale of two serves, with neither man offering barely a sniff of a break. There was only one break point presented to Federer midway through the set, which he couldn't take.

It would take a tie-break to separate the pair and they both lost their opening service points before Federer made the vital breakthrough when Isner served at 6-5 to win the match.

The Swiss next faces eighth seed from Argentina Juan Martin del Potro for a place in the gold-medal final.