Roger Federer staged a dramatic fightback to reach his first Olympic singles final with a record-breaking 3-6, 7-6 (7/5), 19-17 victory over Argentina's Juan Martin del Potro today.

Wimbledon champion Federer was on the brink of a shock defeat on Centre Court after dropping the first set, but the world number one finally subdued del Potro in the longest three-set men's singles match in the Open Era.

The 30-year-old's great escape, in four hours and 26 minutes, means he is guaranteed at least a silver medal in the singles for the first time in his career.

He will face either Novak Djokovic or Andy Murray in Sunday's final.