Roger Federer will contest for an Olympic gold medal after overcoming Juan Martin del Potro in a marathon encounter on Centre Court this afternoon.

Del Potro, representing Argentina, saved a break point in his second service game before taking his chance of a break in the eighth game and serving out the first set to-love 6-3.

Both men were less convincing on serve in the second as they saved numerous break points early on.

In the ninth game of the second it was Federer serving and he moved to 40-0 with ease before wasting game points and almost being broken as Del Potro looked for the killer blow.

The Swiss great just held on though and soon the pair were into a second-set tie-break.

Federer was the first to move ahead in the breaker before Del Potro converted three straight points to level up.

However, it was Federer who won through 7-5 to send the match into a deciding third set.

Federer came under immediate pressure in his opening service game of the third as Del Potro saw two break-point opportunities go begging.

Del Potro himself then had to save break points when serving at 3-3 and 4-4 but managed to come through and stay ahead of the Swiss as the match time reached two-and-a-half hours.

Without a tie-break in the final set, the match passed the three-hour mark as Del Potro showed resolve to hold on at 8-8, producing a diving backhand volley that received a standing ovation from Centre Court.

At 9-9 Federer was presented with yet another break point and this time the Swiss converted for his first break of the match as Del Potro mis-hit a forehand.

Many would have expected Federer to go on and serve out the match but Del Potro's consistent length had the Swiss in trouble and he broke back to-love.

Federer looked in trouble when serving at 11-12 and 0-30 but produced four straight first serves to level up the epic final set once more.

Federer squandered three break points at 14-14 but took his opportunity at 17-17 as Del Potro's backhand found the net.

This time Federer didn't pass up the opportunity to serve it out, just holding his serve to put an end to the contest with the clock at four hours, 29 minutes - the longest tennis match in Olympic history.

The 30-year-old will take on either Great Britain's Andy Murray or Novak Djokovic of Serbia for the gold medal on Sunday.