Complex sentence: The rivalry began highly competitive and remained so for many years however since Murray won the 2013 Wimbledon Championships, Djokovic has taken a commanding lead, winning 14 of their last 17 encounters.
Split into simpler sentences: The rivalry began highly competitive and remained so for many years. However, since Murray won the 2013 Wimbledon Championships, Djokovic has taken a commanding lead, winning 14 of their last 17 encounters.