The seven-time major champion was down a set and 5-3 before staging an impressive comeback, winning 10 of the final 13 games to book her spot in the second round against Alize Cornet.
Buzarnescu takes first set in tiebreak
From the start, both players were striking the ball well with Williams looking to come forward at any opportunity she could and Buzarnescu trying to use her impressive backhand to dictate play.
Neither woman so much as got to deuce until the seventh game when the Romanian had a half-chance on the American's serve only to commit a pair of unforced errors, Williams now ahead 4-3.
She would further extend her lead in the very next game after drilling a backhand crosscourt that the 25th seed was unable to handle. Undaunted, Buzarnescu immediately broke back and the set would be decided in a tiebreak.
The Romanian earned an immediate minibreak with a forehand winner down the line, eventually racing out to a 5-1 lead before taking the breaker 7-3, an ace down the middle clinching a set lead for Buzarnescu.
Williams comes back from the brink to force third set
The second set saw an early trade of breaks with Buzarnescu ahead 2-1. The games were tense and close throughout the rest of the set, Williams looking for any opening on the 25th's seed serve. The sixth game went to deuce, but the American could make no further progress.
Buzarnescu struck what looked to be the decisive blow in the eighth game. Ahead 15-40 on the Williams serve, she wasted one break point, but was gifted the break when the American double-faulted to give the 25th seed a 5-3 lead.
Serving for the biggest win of her career, the Romanian blinked, broken to love and, much like the first set, the middle frame would head to a tiebreaker.
After each player twice traded minibreaks, Williams ran off the final four points, aided by a pair of unforced errors by Buzarnescu and the match, which seemed destined to be won by the Romanian, was headed to a final set.
Seven-time major champion dominates third set
Williams got off to the perfect start in the final set, holding to love before battling through a long game on Buzarnescu's serve that saw the Romanian double-fault on break point to hand the American a 2-0 lead.
The 25th seed was playing too well not offer a fightback and she created an opening at 15-30 on the Williams serve, but the world number 36 showed her championship resolve, digging out a hold with some timely forehands.
The ability of Williams to move forward paid off in the fifth game as she saved a break point with a backhand crosscourt followed by a volley winner, eventually holding for a 4-1 edge.
Both women saved break points in their subsequent service games with the American on top 5-2. Serving to stay in the match, Buzarnescu failed on a backhand slice to give Williams two match points. The American capitalized with a backhand drive that sealed her passage to the next round.
Williams meets the media following tough victory
Following the win, Williams met the media and began by saying: "My opponent was playing really well. There are no easy matches here. I was really happy to play to be able to play better than an opponent who is really in form.
"I really thought I played well today. I thought my opponent played really well. Those sorts of matches aren't easy where you don't have a lot of openings, where you have to try and create something and see what happens."
She talked about what she was going through her mind down 5-3 in the second set: "There were moments where it looked like maybe she had the match at that time. I kind of just forgot the score and kind of focused on my game.
"That's what great about tennis: the clock doesn't run out".