The match in Abu Dhabi finished 6-2, 3-6, 10-5 with a "super tie break" deciding matters in the final set.
Williams was making her comeback after a year-long layoff to have a child.
What a match! ?— Jelena Ostapenko (@OstapenkoJ1997) December 30, 2017
After a decider tie-break, Alona defeats the 23-time Grand Slam champion @serenawilliams 6-2, 3-6, [10/5] in @MubadalaTennis exhibition ??#MWTC10 pic.twitter.com/OI1boYVsUT
Ostapenko is on the record as saying Serena is one of her tennis role models, making the victory in three sets even more significant to the rising Latvian star.
Meanwhile in Brisbane Anastasija Sevastova also started the new season with a win.
Anastasija Sevastova was impressive in her win over home favorite Stosur at @BrisbaneTennis!— WTA (@WTA) December 31, 2017
HIGHLIGHTS --> https://t.co/oVJbUJqDwg pic.twitter.com/bjVKMZxiee