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Kerri Walsh Jennings isn't accustomed to losing at the Olympics -- not even a set.

She was put in a rare position on Tuesday when she and partner April Ross dropped the second set of a match against Switzerland and were forced to rally in a third-set tiebreaker.

The 37-year-old Walsh Jennings -- going for her fourth straight Olympic gold -- had only lost one set in Summer Games competition before Wednesday night.

Walsh Jennings and Ross rallied in the third set, however, to win 21-13, 24-22, 15-12 and advance to the Round of 16.

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Kerri Walsh Jennings and April Ross pulled out a close win on Wednesday. Getty Images

The Swiss duo of Isabelle Forrer and Anouk Verge-Depre gave the Americans their toughest test so far in Rio but ultimately fell short. They'll have to wait until pool play ends to see if they qualify for the elimination round.

Walsh Jennings and Ross also beat Australia and China en route to a 3-0 record in their group.

This is the first Olympic Games where Walsh Jennings is paired with Ross. Misty May-Treanor, Walsh Jennings' partner for the three prior golds, retired after the 2012 London Olympics.