Once these two giants have completed their game Scotland's Andy Murray must once more face Rafael Nadal. Fourth-seed Murray has cruised to the semi-finals stage despite being forced to have no rest days in since Tuesday as US Open organisers battled earlier in the week with heavy rain and cracks on the centre court.
Andy Roddick's match with David Ferrer had to be moved to a different court after play was stopped when water seeped through the surface
in a fourth-round clash.
Andy Murray has already bemoaned the "nightmare weather" conditions that have plagued this tournament. Indeed the rain has started falling again just as Federer is preparing to line up against Djokovic.
The women's semi-finals, normally played and completed by Friday, will attempt to get under way during Saturday evening on the Grandstand court where Samantha Stosur will face Angelique Kerber with Serena Williams involved in her tie against Caroline Wozniacki on the Arthur Ashe Court.
The women's final has now been rescheduled to play on Sunday at 4pm (EDT) and the men's final has now been moved to Monday at 4pm (EDT).
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. Small pockets of blue sky appeared over Flushing Meadows but there could be even more delays
as the ground has to dry out first.