Serena and Venus Williams first faced each other professionally in 1998.
Their first professional singles match took place in the second round of the 1998 Australian Open in Melbourne. Serena, then 16, had just defeated sixth-seeded Irina Spîrlea in her first Grand Slam main-draw match. Venus won the sibling matchup in straight sets after taking the opening set in a tiebreak.
The meeting was historically unusual because it pitted two sisters against each other at the highest level of professional tennis. Venus was already the more established player, and she advanced while Serena gained experience from her first major tournament. Their rivalry would later include Grand Slam finals, including the 2002 French Open and 2002 US Open.
This answer refers to their first professional singles face-off. The sisters had already appeared together as a doubles team before 1998, so “first faced each other professionally” means their first competitive match as opposing singles players.