The correct answer is: Rafael Nadal.
Rafael Nadal won the 2017 Madrid Open title by defeating Dominic Thiem in the final. Nadal won the match in straight sets, 6-4, 6-3. This was Nadal’s sixth Madrid Open title, and his 32nd ATP Masters 1000 title overall.
Kavin Anderson is a South African professional tennis player. He has won one ATP Tour singles title, and has reached a career-high ATP singles ranking of world No. 52. Anderson has never won a Masters 1000 title.
Juan Martin del Potro is an Argentine professional tennis player. He has won 22 ATP Tour singles titles, including one Grand Slam title at the 2009 US Open. Del Potro has reached a career-high ATP singles ranking of world No. 3. He has never won a Masters 1000 title.
Roger Federer is a Swiss professional tennis player. He has won 20 Grand Slam singles titles, the most in history for a male player. Federer has also won a record 31 ATP Masters 1000 titles. He has reached a career-high ATP singles ranking of world No. 1. Federer has never won the Madrid Open title.