Corentin Moutet heard the boos after snapping his racquet and retiring in Madrid.
The Greek says she needed a win like today's, for her confidence.
Two-time US Open champion Andre Agassi will play a different sport's US Open this weekend.
"It took me as long as it took because I needed time to be convinced that the decision I was making was right," Rafa Nadal said of his retirement.
Rune is back in the Top 10 after a year outside.
Djokovic could meet his Spanish rival in the semis.
The Pole is defending champion at the Madrid Open.
