Answer
Fabio Fognini's first top 10 scalp was Roger Federer. Fognini defeated Federer in the third round of the 2011 Monte Carlo Masters. The match was a close one, with Fognini winning 7-6, 6-7, 7-5. This was a major upset, as Federer was the defending champion and one of the top players in the world. Fognini's victory showed that he was capable of beating the best players in the world.