Serena Williams and Victoria Azarenka met in the final at the Western & Southern Open in Cincinnati earlier this month. Azarenka has beaten Williams in two of three meetings this season. (Matthew Stockman/Getty Images)

The key to any tennis champion’s longevity is avoiding injury and discovering new motivation.

No women’s player has mastered that delicate dance in recent decades better than Serena Williams, 31, who has outlasted such rivals as Martina Hingis, Justine Henin, Kim Clijsters and Amelie Mauresmo, all former No. 1 players and contemporaries who have bowed out of the pro tour’s debilitating grind.

As the U.S. Open gets underway Monday in Flushing, N.Y., Williams, its four-time and defending champion, has found her latest motivation in a player seven years her junior, Victoria Azarenka of Belarus, who has emerged over the past year as the American’s most formidable rival, though their lopsided career record, which Williams leads 12-3, suggests otherwise.

But Azarenka, who was just 6 when Williams turned pro, has won two of their three meetings this year, including a 2-6, 6-2, 7-6 (8-6) battle-of-wills earlier this month in Cincinnati.

Speaking to reporters over the weekend, Williams all but thanked Azarenka for the rare defeat, intimating that the Belarusian could expect a tougher fight if the two meet in the U.S. Open’s final for a second consecutive year, as their No. 1 and 2 seeds suggest.

“Every time I lose, I get so pumped afterwards,” Williams said of her Aug. 18 loss to Azarenka, which snapped her 14-match winning streak. “I just feel like now I’m ready; now I’m prepared. I almost needed that to take my game to a new level.”

Women’s tennis could use the spark of a new rivalry.

Apart from Williams’s elder sister Venus, few players have been able to handle the power of Serena Williams’s game.

Maria Sharapova can. She also can match Williams’s formidable mental toughness. But Sharapova lacks the variety of strokes and tactics to keep Williams off-balance. Instead she returns power with power, which tends to feed into Williams’s strong suit. And like many others, Sharapova struggles with Williams’s serve, considered the best in the women’s game. The upshot: Williams holds a 14-2 advantage in matches against the 6-foot-2 Russian, who pulled out of this year’s U.S. Open with bursitis in her surgically repaired right shoulder.

The 6-foot Azarenka presents a bigger challenge. She hits the ball every bit as hard and fights with the same ferocity that Williams and Sharapova do. But she’s quicker on court than Sharapova and isn’t afraid to vary her tactics — even charge the net — in an effort to salvage a set that’s slipping away.

More importantly, Azarenka is getting better at controlling her combustible emotions, which often derailed her as a younger player.

The mental game is largely what failed her in last year’s U.S. Open final. Leading 5-3 in the third set, Azarenka was gripped by an ill-timed bout of anxiety, lost her serve and allowed Williams to seize control and storm back for the victory.

Azarenka insists the defeat doesn’t weigh on her. Like the best NFL cornerbacks, she says she moved on at once, refusing to let one bad episode drag her down.

“I never look back, really,” Azarenka said during an interview Saturday. “I always look forward. My head doesn’t spin all the way back.”

Azarenka enters the U.S. Open more rested than Williams, having played only 40 matches. Williams has played 64, posting a 60-4 record. Two of those four losses were to Azarenka.

It’s a statistic that Chris Evert, whose Hall of Fame career lasted 17 years, predicts Williams will draw on to exact revenge.

“She’s a better player when she gets mad at herself,” Evert said of Williams. “When she gets too calm, she gets a little complacent. If she’s winning too easily, she gets complacent.

“I think more doubts are in her subconscious now. Every time you lose to a player, you do have a doubt. Even if it’s not conscious, it could be subconscious and affect her play.”