By proposing to him, apparently. Thanks to Yahoo! Sports for the news:
During the world No. 2’s second round match at the Aussie Open a woman in the stands yelled, “Hey, Nadal, will you marry me?”, much to the delight of others in the crowd and the Eurosport announcers broadcasting the match. At first Nadal ignored the question, probably wanting to stay focused on returning serve, but as the laughter increased he gave a shy wave of the hand and then burst out into a full smile before the point.