The 23-time Grand Slam winner needed over two hours to see off the challenge of the Romanian. The win set up an all-American third-round meeting with last year's quarterfinalist Danielle Collins, who might subject Serena to a stern test.
On that note, let's take a look at the three things we learnt from Serena Williams' win over Buzarnescu:
#1 Serena Williams keeps fluctuating from magical to mystifying
It's quite evident that Serena Williams is struggling to maintain a high level for long periods of time. Her oscillation from brilliant to sloppy almost proved costly in this match.
Serena carved out break points in the fifth game of the second set but failed to capitalize on her chances and dropped her own serve for the first time in the match in the very next game. The American came back from a 0-40 hole by dint of her big serves but backhand errors handed the advantage back to her opponent.
Again, with Buzarnescu serving at 5-5, Serena had two more opportunities to break. Once more she was unsuccessful and then went on to concede the set 7-5.
But just like the Serena of yore, she was able to regroup and build a big lead in the decider. The American raced to a 3-0 lead and did not let up as she dismantled Buzarnescu 6-1 in the final set.
#2 Drawing Serena to the net might be a good idea
Serena Williams likes to control proceedings from the baseline and moving to the net isn't her cup of tea. Quite often in this match, the American looked unsure and tentative whenever she went to the forecourt.
Buzarnescu cashed in to earn a few crucial points in the second set. The southpaw kept slicing the ball to force Serena to move forward and then passed the American at the net.
Buzarnescu saved a break point with this approach while serving in the first game of the set and then repeated the feat in the fourth and fifth games as well.
The fact that Serena won just 50% (6/12) of her net points is evidence of her struggles.
The World No. 8 is now almost 40 and her speed and reflexes simply aren't what they were before. Testing her movement might be a good tactic whenever her opponents need to wrest control from the American.
#3 Serena wins two consecutive matches on clay for the first time this season
Serena Williams didn't enter Roland Garros with the ideal preparation. Having seen her season marred by oral surgery, the American has found it tough to get going.
In the two claycourt tournaments leading up to the French Open, Serena won a grand total of just one match.
Despite her struggles, she remains as determined as ever and has now notched up two more wins at the highest level of the sport, taking her Grand Slam match win tally to 364.
If Serena can get through the first week unscathed, she will fancy her chances of lifting the Coupe Suzanne Lenglen for the fourth time in her career.