Serena Williams in QFs of the Mutua Madrid Open 2015.

Though Williams had won 14 of 17 previous meetings against Azarenka, and both clay court meetings in straight sets, many of them had been close, and this one was arguably the closest. The pair battled it out for two hours and 43 minutes until Williams finally emerged with the victory, 7-6(5), 3-6, 7-6(1). 

 And though she only came away with the middle set, Azarenka could have easily won any of the other sets - she had a massive 5-1 lead in the first set tie-break, and she served for the match at 6-5 in the dramatic 69-minute third and deciding set, even bringing up 40-0 and triple match point in that game. But the No.1-seeded Williams fought them all off, the first one with a forehand winner, the second one with an unretrievable backhand return and the third one off an Azarenka double fault. In fact Azarenka double faulted three times in a row to give Williams the break back, and she cruised from there.