MELBOURNE, Australia — As Serena Williams made her way out to the Australian Open’s main court during pre-match introductions, the voice booming over the stadium’s speakers implored spectators to welcome “the world’s No. 1 player — from Romania, Simona Halep.” Oops. Williams, wearing headphones, quickly pivoted and retreated back into the tunnel she’d just emerged from, making way for her fourth-round opponent Monday night.