U.S. heir apparent to Serena Williams is ...

Two years ago, those rueing the demise of American men’s tennis could take some comfort in the fact that the U.S. women were still a force in the upper reaches of the game.

Serena Williams was queen of all she surveyed, and -- better yet -- it seemed that she would have a worthy, if not exactly comparable, successor. In August 2012, skillful 20-year-old newcomer Christina McHale was knocking on the door of the top 20. By early January 2013, Varvara Lepchenko hit No. 21 in the rankings. A few weeks later, highly touted Sloane Stephens upset Serena in the quarterfinals of the Australian Open.

Stephens, then 19, would go on to make the Sweet 16 (fourth round) or better at every other Grand Slam event as well that year. And there were other promising players in the top 100, including Jamie Hampton, Coco Vandeweghe and Lauren Davis.

Now, two years later, the closest thing the U.S. has to a legitimate successor to Serena Williams is -- at least going by the Week 1 results on the WTA Tour -- Venus Williams. That’s right. Venus, 34, a winner last week in Auckland. Serena’s older sister. Can your older sibling be your “successor"? I sure hope so, because if it weren’t for that possibility, it appears we would have none at all.

While Venus was busy winning Auckland, Stephens was losing in the same event to Davis, who then lost to ... Venus. It was fitting to the point of symbolism. If you’re from the U.S., you’d better hope the Williams sisters can play singles into their 40s or 50s. Maybe beyond.

Right now, Serena is ranked No. 1 and Venus is No. 18. Lepchenko, one of the few bright spots last week (she made the semis in Brisbane), is next at No. 30. Stephens is down to No. 34, and McHale is languishing at No. 53. OK, it’s time to bring in some new, potential inheritors -- this time a trio that is moving in the right direction: Madison Keys, Vandeweghe and Alison Riske.

Keys is just a few ticks behind Lepchenko, a 28-year-old naturalized citizen (native of Uzbekistan) at No. 33. That’s already one ranking place better than Stephens, while Vandeweghe is breathing down Stephens’ neck at No. 37, and Riske also is in striking distance at 42.

Among these women, Keys appears to have the most upside. Just 19 years old, she’s now working with a Hall of Fame player (and fellow countrywoman) with whom she has much in common as an athlete: Lindsay Davenport.

Davenport may be 4 inches taller, but Keys is no shrimp at 5-foot-10. She’s also built on a similar, large frame. According to Keys, she and Davenport have already done some valuable work on an issue of major concern for almost every big player: movement. As if to punctuate their success, Keys upset Dominika Cibulkova -- one of the best movers on the tour -- last week in the first round of Brisbane.

Keys is also trying to master that champion’s knack for winning when she isn’t playing her A-game. As she said after her win over Cibulkova: “So I think just having a more consistent game, and when I'm not playing my best, having a B-game or C-game -- that isn't terrible. [Whether] I'm playing really well or I'm playing badly, [I’m] trying to find a middle ground for the days where it's not working."

Like Keys, Vandeweghe won her first WTA title on grass last year. (Keys won at Eastbourne, Vandeweghe at ‘s-Hertogenbosch.) Just 23 and a gifted athlete, Vandeweghe was outside the direct-acceptance ranking at the end of 2013 (No. 110), but she quietly vaulted all the way to No. 38 through the course of last year.

At 6-foot-1 and 155 pounds, Vandeweghe is even closer to the Davenport model than Keys. She’s explosive and armed with a dangerous serve, eventual strengths that may have held her back while she was still growing and struggling to make all those moving parts act in concert. But Vandeweghe has matured and become more consistent. It seems her game and body are in greater sync now.

Riske, 24, more or less came out of nowhere -- deciding at the last moment to pass up a college scholarship in 2009 in favor of trying her hand on the WTA Tour. A hardworking, independent-minded player of 5-foot-9, Riske made her breakthrough in 2013. She bolted from No. 179 to No. 57. This is a player who will wring every drop of useful information out of her experience, and she isn’t afraid of big occasions. She made the third round at Wimbledon and the Australian Open last year.

Heading into the Australian Open, one of these three women seems most likely to join Venus and Serena at the helm of U.S. women’s tennis for 2015 -- unless Stephens can pull herself out of what has become a long, nightmarish slump (compounded by a wrist injury that ended her 2014 campaign in mid-September). McHale lost the only match she’s played this year and has trouble with her shoulder.

In real monarchies, queens do not retire. They are the national figureheads for life. If you’re an American, you probably wish it were that way in tennis, too.