BASEL, SWITZERLAND - OCTOBER 29:  Marin Cilic of Croatia in action during the Swiss Indoors ATP 500 tennis tournament semi-final match against Mischa Zverev of Germany at St Jakobshalle on October 29, 2016 in Basel, Switzerland.  (Photo by Harold Cunningham/Getty Images)
BASEL, SWITZERLAND - OCTOBER 29:  Marin Cilic of Croatia in action during the Swiss Indoors ATP 500 tennis tournament semi-final match against Mischa Zverev of Germany at St Jakobshalle on October 29, 2016 in Basel, Switzerland.  (Photo by Harold Cunningham/Getty Images)
BASEL, SWITZERLAND - OCTOBER 29:  Marin Cilic of Croatia in action during the Swiss Indoors ATP 500 tennis tournament semi-final match against Mischa Zverev of Germany at St Jakobshalle on October 29, 2016 in Basel, Switzerland.  (Photo by Harold Cunningham/Getty Images)