Complex sentence: The rivalry was rekindled during the 2013--14 season when the Nets and Raptors battled for the Atlantic Division, which the Raptors won but then they found themselves facing each other in the opening round of the 2014 NBA Playoffs.
Split into simpler sentences: The rivalry was rekindled during the 2013--14 season when the Nets and Raptors battled for the Atlantic Division. The Raptors won the division final but then they found themselves facing the Nets in the opening round of the 2014 NBA Playoffs.