* Mickelson opponent injured on final hole * McIlroy also remains perfect (Updates at end of play, adds details, quotes) March 24 (Reuters) - Jordan Spieth and Phil Mickelson won their respective matches at the WGC-Dell Match Play in Texas to retain perfect records in contrasting fashion, while Jason Day overcame a back injury to join them with a victory on Thursday. On a day that also saw third seed Rory McIlroy win to improve to 2-0-0, world number one Spieth oozed confidence with the state of his game at Austin Country Club. "Very pleased to get today done with quickly," top seed Spieth told Golf Channel after dusting off Frenchman Victor Dubuisson 5&4. More...