For as long as Serena Williams remains in the U.S. Open bracket, there is one overwhelming question that will hang over the proceedings and help define the moment: Could this be the last chance to watch her play?

The second opportunity for spectators to shower Williams with adoration — and for the 23-time major champion to enjoy that outpouring at what she has hinted, but purposely not definitively declared, will be her final tournament — began Wednesday night in Arthur Ashe Stadium, the largest arena in Grand Slam tennis.

The opponent this time: No. 2 seed Anett Kontaveit, a 26-year-old from Estonia whose current ranking is much better than Williams’ is, but whose career resume lacks so much as one quarterfinal victory at any Slam event.

Williams won the first set in a tiebreaker by a 7-6 (4) score, closing it out with an ace after 1 hour, 3 minutes. Kontaveit quickly righted herself, racing to a 3-0 lead in the second and eventually grabbed it 6-2 to force a third.

About 1 1/2 hours before the scheduled start of the match, Williams went out to the practice courts for a 30-minute warmup session. She crossed paths with her sister, Venus, who was wrapping up some doubles work on that same court.