Serena Williams was cruising toward her 16th Grand Slam tournament championship, another notch in her belt on the way to catching Martina Navratilova and Chris Evert with their 18 titles -- and maybe even Helen Wills (19), Steffi Graf (22) and Margaret Court (24) beyond them.

But while a nasty ankle injury during a 6-0, 6-0 first-round win didn't stop Williams at the Australian Open, back spasms in her quarterfinal match Wednesday night in Melbourne absolutely did. With her fearsome serve -- the one she used to blow No. 14 seed Maria Kirilenko off the court two days earlier -- neutered, the intimidation factor was gone against 19-year-old American Sloane Stephens. And so a young player who looked shell-shocked as her own round of 16 match two days earlier against Bojana Jovanovski slipped away suddenly found herself in the semifinals.

Stephens deserves some credit. She rallied in that third set against Jovanovski to secure a 6-1, 3-6, 7-5 victory and earn the chance to face Williams in the first place. And then she took full advantage when Williams pulled up lame with that sore lower back in the second set of a 3-6, 7-5, 6-4 victory.

But it's unlikely anyone was beating a healthy Williams at this tournament. She hadn't lost a match anywhere since August, and she had handled Stephens relatively easily at a tournament in Brisbane the week before. Yes, No. 1 seed Victoria Azarenka and No. ?2 seed Maria Sharapova both reached the semifinals. But Williams beat Azarenka in the U.S. Open final in September and had beaten Sharapova nine consecutive times, including in the Australian Open final in 2007 and at Wimbledon in 2010.

In reality, the only thing that can keep Williams, 31, from dominating at a high level for another few years is her body. She has long fought off foot and ankle issues. A sore back would be another burden. And while she has plenty of time to recover before the French Open in May, this still felt like an opportunity taken from her. It would have been Williams' sixth Australian Open title.

- Brian McNally