Complex sentence: Federer lost to Nadal once more at the French Open in 2011 which to date is the only Grand Slam final they have met in since, although Federer lost to Nadal twice more at the Australian Open semifinals in 2012 and 2014.
Split into simpler sentences: Federer lost to Nadal once more at the French Open in 2011 which is the only Grand Slam final they have met in since. Federer also lost to Nadal twice more at the Australian Open in semifinals in 2012 and 2014.