Opening encounters are a fundamental component of every interaction. Psychology research highlights the valence of opening encounters as one of the main factors shaping the interaction that follows. We evaluated whether opening encounters would have a similarly powerful effect on human-robot interactions. Specifically, we tested how positive and negative opening encounters with a robot would impact the subsequent interaction. Participants interacted with a robot that performed gestures communicating different valences of opening encounters: Positive, Negative, or No opening encounter. To evaluate the impact on the subsequent interaction, we measured participants' willingness to comply with a help request presented by the robot and their perception of the robot. Our results indicated that most participants in the Positive opening encounter condition helped the robot and described a positive overall perception. An opposite pattern emerged in the other two conditions. Almost none of the participants helped the robot, and the perception of the robot was less positive. Our findings suggest that opening encounters with robots should be carefully considered due to their impact on the interaction that follows.