The American team has won the last two Olympic titles and did not lose a set while doing so.
May-Treanor and Walsh-Jennings had to rally from 3 points down in the second set of their match Monday, but in Wednesday's contest they suffered a 21-17 setback to start their matchup with sisters Stefanie and Doris Schwaiger of Austria.
The United States duo, already assured of a spot in the knockout round, then blitzed the Austrian team 21-8 in the second. The third and final set of a beach volleyball match is played to just 15 points and the Americans got to that total in only 12 minutes while winning 15-10.
May-Treanor and Walsh Jennings thus finished 3-0 in Group C.
Sixteen teams will begin the knockout round Friday, with the quarterfinals set for Sunday.