The 26-year-old Australian, who had scored a wildcard entry to the claycourt grand slam starting on Sunday, put up a fight on Wednesday before going down 1-6, 6-0, 7-5 to Morita in the second round.
Dellacqua reached the third round at Roland Garros in 2008, but her ranking had since slipped to 431st, with her career taking a hit due to serious shoulder and foot injuries.
Serbian fourth seed Jelena Jankovic also had her French Open preparation cut short, suffering an upset 3-6 6-3 6-3 loss to Sweden's Sofia Arvidsson in Brussels.
"Jelena and I have had battles before and I had match points last time, so I really wanted to win this time," Arvidsson said.
"Usually you play the big players on the big courts, but today we were on a smaller court, so maybe that was an advantage for me. Anyway, I'm happy to win today and get my revenge."
The Swede next faced Chinese eighth seed Peng Shuai, who eased past Abigail Spears of the United States, 6-2 6-0.
"The last time I played Peng was in Fed Cup last year and I had eight match points and lost, so it's another chance for revenge. She's playing well and having a good season, so it will be tough," Arvidsson said.
Second seed Vera Zvonareva had to battle back from a first set loss to see off Kazakhstan qualifier Galina Voskoboeva 2-6, 6-4, 6-3.
"It was my first time playing here and I had to get used to the conditions, new court, new balls, new atmosphere," Russian number one Zvonareva said.
"Not only was I a little bit slow at the start but she was playing well. She qualified here so she was used to the conditions.
"But I found my rhythm and got better as the match went on. Now I have another chance for another match here."
Top seed Caroline Wozniacki rallied from a break down in both sets, finally beating American Varvara Lepchenko on her fifth match point, 6-4 7-6 (7/5).
- AAP/AFPTags: sport, tennis, belgium First posted May 19, 2011 13:23:00