“I thought I did a really good job in the first set and a half, but that’s obviously not enough to win the match,” Sharapova said.
“I did a good job of winning the longer rallies, even though that’s not really what I wanted to get myself into. So physically I felt good.
“I just got pretty passive in the end and starting making too many errors.” Sharapova was downed 4-6, 6-4, 6-3 by former world No 28 Niculescu and hit a staggering 52 unforced errors during the match.
The defeat comes after she crashed out in the third round of the Australian Open but was adamant she knows how to put things right.
“I love playing in three-set matches, and I think I’ve thrived in them my career,” she said. “So I don’t necessarily shy away from that challenge.
“I think I have to look at the tape and see why I started going back behind the baseline and not moving forward, not putting the pace on her.”
Earlier, Dominika Cibulkova rallied past Russia’s Anastasia Pavlyuchenkova 7-6(8) 6-4 as she progressed to a second round meeting with France’s seventh seed Caroline Garcia.
Romania’s Mihaela Buzarnescu eased past Lesia Tsurenko to set up a clash with sixth-seeded French Open champion Jelena Ostapenko, while American qualifier CiCi Bellis advanced after Daria Kasatkina pulled out of their opener with an injury.
Russians Ekaterina Makarova and Anna Blinkova also reached the second round.
Second Round Results
7-Caroline Garcia (FRA) beat Dominika Cibulkova (SVK) 6-3 6-7(3) 6-0 4-Garbine Muguruza (ESP) beat Ying-Ying Duan (CHN) 6-3 6-4
Carina Witthoeft (GER) beat Tatjana Maria (GER) 6-3 6-0 14-Magdalena Rybarikova (SVK) beat Fatma Al Nabhani (OMA) 6-3 6-1
10-Johanna Konta (GBR) beat Bernarda Pera (USA) 7-6(5) 6-1 Carla Suarez Navarro (ESP) beat Kateryna Bondarenko (UKR) 3-6 6-3 6-3 Samantha Stosur (AUS) beat Irina-Camelia Begu (ROU) 6-4 6-2 Marketa Vondrousova (CZE) beat Yulia Putintseva (KAZ) 6-3 6-1 16-Petra Kvitova (CZE) beat Cagla Buyukakcay (TUR) 6-0 6-3 9-Julia Goerges (GER) beat Lucie Safarova (CZE) 6-1 7-5 Barbora Strycova (CZE) beat Aleksandra Krunic (SRB) 6-4 6-4 11-Kristina Mladenovic (FRA) beat Shuai Peng (CHN) 6-2 6-4 15-Elise Mertens (BEL) beat Timea Babos (HUN) 2-6 6-1 6-1 Sorana Cirstea (ROU) beat Maria Sakkari (GRE) 6-2 6-3 Alize Cornet (FRA) beat Anett Kontaveit (EST) 6-2 6-1 12-Madison Keys (USA) beat Qiang Wang (CHN) 6-1 6-4 13-Anastasija Sevastova (LAT) beat Donna Vekic (CRO) 6-3 6-4
1-Caroline Wozniacki (DEN) (Bye) 8-Angelique Kerber (GER) (Bye) 3-Elina Svitolina (UKR) (Bye) 6-Jelena Ostapenko (LAT) (Bye) 7-Caroline Garcia (FRA) (Bye) 4-Garbine Muguruza (ESP) (Bye) 5-Karolina Pliskova (CZE) (Bye) 2-Simona Halep (ROU) (Bye)