The 19-year-old, ranked 41 in the world, lost 6-2 7-5 to the Japanese player ranked two places below her.
Robson struggled with her serve and had 10 double faults in the 68 minute reverse.
The Londoner has struggled since reaching the third round of the Australian Open in January, losing in the opening round to lower ranked opponents in five tournaments and mustering just two wins.
WTA Estoril results:
4-Carla Suarez Navarro (Spain) beat Marina Erakovic (New Zealand) 6-3 6-0
5-Sorana Cirstea (Romania) beat Anabel Medina Garrigues (Spain) 6-2 2-6 6-1
Svetlana Kuznetsova (Russia) beat 6-Varvara Lepchenko (U.S.) 6-7(2) 6-3 6-2
7-Elena Vesnina (Russia) beat Maria Joao Koehler (Portugal) 6-4 6-1
Ayumi Morita (Japan) beat Laura Robson (Britain) 6-2 7-5
Yanina Wickmayer (Belgium) beat Anna Tatishvili (Georgia) 6-1 7-5
ATP Estoril results:
6-Benoit Paire (France) beat Igor Sijsling (Netherlands) 7-6(5) 6-3
Denis Istomin (Uzbekistan) beat Evgeny Donskoy (Russia) 2-6 7-6(6) 6-4
David Goffin (Belgium) beat Pedro Sousa (Portugal) 7-6(1) 4-6 6-4
Edouard Roger-Vasselin (France) beat Gilles Muller (Luxembourg) 6-1 (Muller retired)
ATP Munich results:
Marinko Matosevic (Australia) beat Kevin Krawietz (Germany) 6-2 6-4
Daniel Brands (Germany) beat Tobias Kamke (Germany) 5-7 6-4 6-3
Dmitry Tursunov (Russia) beat Paul-Henri Mathieu (France) 6-3 6-4
Ivan Dodig (Croatia) beat Nikolay Davydenko (Russia) 6-4 6-4