Serena Williams (USA #1) vs.  Li Na (CHN #5)
Head-to-head: Williams leads 6-1
At a set and 4-1 down to Dominika Cibulkova in the fourth round on Monday, Serena Williams looked on the verge of making her earliest exit at Crandon Park since 2000. Somehow, though, she rescued the situation, eventually prevailing 26 64 62 to score her 58th win at the tournament.
Williams, a five-time Miami champion, is now just one shy of Steffi Graf's all-time record of 59, but the history books will be the last thing on her mind when she steps out for her blockbuster quarterfinal with Li Na.
Despite her miserable head-to-head record - the last time she took a set from Williams was at the same stage in Miami four years ago - Li will have seen enough vulnerabilities against Cibulkova to believe an upset is possible. Further encouragement comes in the shape of her own form, which has seen her negotiate the opening three rounds without dropping a set.
 Agnieszka Radwanska (POL #4) vs.  Kirsten Flipkens (BEL #28)
Head-to-head: Radwanska leads 1-0
This time last year, Kirsten Flipkens was in Thailand playing a $25K tournament on the ITF Circuit as she worked her way back up the rankings after a troubled 2011. Twelve months, 56 wins and a Top 30 debut later, Flipkens is in the quarterfinals of one of the biggest events on the WTA calendar, taking on the defending champion, Agnieszka Radwanska.
And while Flipkens will certainly be the underdog, she proved in running Victoria Azarenka close in Indian Wells and again during her third round win in Miami over Petra Kvitova that she can certainly hold her own against the game's elite.
Flipkens displayed a pleasing mixture of court craft and aggression to befuddle both Azarenka and Kvitova, with her backhand proving a particularly potent - and varied - weapon. In Radwanska, she is coming up against a kindred spirit, who, as illustrated in the picture-book perfect final set and a half against Sloane Stephens, is loathe to play second fiddle to anyone in the shot making department.