Answer
Rafael Nadal was defeated by Jo-Wilfried Tsonga in the fourth round of the 2008 Australian Open. Tsonga, ranked 38th in the world at the time, upset the second-seeded Nadal in four sets, 6-2, 6-3, 6-2. It was a major upset, as Nadal had won the Australian Open in 2009 and was considered one of the favorites to win the tournament in 2008.