Over the past few years, tennis fans have been privy to otherworldly levels of greatness. On the men's tour, the torch of invincibility has been passed around from Roger Federer to Rafael Nadal to Novak Djokovic. Today it's firmly in Djokovic's hands: the Serb has made the finals of the past 16 events he's played, winning 12 of them.

On the women's tour, it's been Serena Williams vs. The Field for some time; the 21-time Grand Slam champion has won four of the past five majors and has almost twice as many rankings points as the world No. 2, Simona Halep.

While they're the overwhelming favorites in their respective draws, Djokovic and Williams are not the most dominant players Down Under. That honor belongs solely—or, rather, jointly—to the powerhouse women's doubles team of Martina Hingis and Sania Mirza.

Hingis and Mirza—or, as Twitter has taken to calling the pair, #SanTina—are currently on a 31-match, seven-tournament winning streak, the longest of its kind in women's doubles since Jana Novotna and Helena Sukova won 44 in a row back in 1990. The duo have won 11 tournaments altogether, including Wimbledon, the U.S. Open, and the WTA Finals, since they first partnered with each other just 10 months ago.

Along the way they've brought each other's already-illustrious careers to new heights. Mirza, the Indian feminist icon, was a top-five doubles player and three-time Slam champion in mixed doubles before partnering with Hingis, but she'd never been ranked No. 1 or won a women's doubles Slam. It took her just three tournaments with Hingis to accomplish the former, and 10 to nab the latter.

Hingis, meanwhile, has been able to add so much gravy to her Hall of Fame resume that they might have to induct her all over again. Since returning to the tour for doubles in 2013, she has added three mixed doubles majors, two women's doubles majors, and 18 women's doubles titles to her haul, making her one of the most decorated champions of all time. Most of that success has come with Mirza.

At this point in their careers, both 29-year-old Mirza and 36-year-old Hingis are talented, but together, they're simply transcendent.

"The chemistry is amazing. We don't always play super tennis, but we come out and find a solution," Hingis said last week. "I think that's what makes us right now the best doubles in the world."

Women's doubles has been called "the best-kept secret in sports."

Whereas singles tennis is often compared to boxing, with two individuals trading blows in a battle as much about wills as it is about skill, doubles is much more intricate and delicate. It's fast-paced and frenetic, emotional and interactive. In most cases, it's also a complete disaster—a chaotic cacophony of groundstrokes and (mis)communications. But when lightning strikes, when two players are completely in-sync mentally, physically, and emotionally for a point or a game or a match or a season, it's absolutely electrifying.

According to legendary coach Nick Bollettieri, "the No. 1 rule when playing doubles tennis is to find the right partner."

Clearly, Hingis and Mirza have accomplished that task, albeit mostly by chance. Around this time last year, both stars were foundering in unhappy doubles partnerships. Hingis had been having some success with Flavia Pennetta, but the Italian was ready to bow out of doubles and focus on her singles career. Mirza, meanwhile, had been playing with Su-Wei Hsieh since her previous partner, Cara Black, retired at the end of the 2014 season. But there was a big problem: both Mirza and Hsieh preferred to play from the deuce (right) side of the court, and neither were very comfortable switching to the ad side.

So, at a tournament in Doha last February, Hingis and Mirza took the opportunity to practice together. Even though that first practice went terribly, and Mirza ended up making it to the final with Hsieh, the two decided to go ahead and partner up anyway. Their decision paid off quickly: #SanTina won their first three tournaments.

"Coming to [Hingis], she's obviously one of the best players out there, and most importantly one of the best people who can complement the way I play," Mirza said. "She's got amazing—probably one of the best hands in the world at the net. I need someone who can finish the balls off where I set them up."

Indeed, their styles are a perfect fit. Hingis still has the net game, speed, and court sense that helped her win three straight majors at age 16. She just doesn't have the power or athleticism to keep up with the modern game, which is where Mirza and her lethal forehand comes in. Together, they have no weaknesses, only jaw-dropping, opponent-crushing strengths.

When tennis writer Stephanie Neppl first heard about the union, she was instantly intrigued.

"I thought, That is the best doubles pairing I think I've ever heard of on the women's side," she said.

It wasn't quite supposed happen this way. Hingis and Mirza have both had their fair share of detours.

"I remember when Hingis was 15 and won the doubles at Wimbledon. She was this mousey-haired, smiley girl—really a teenage dream," Neppl said. "Then two months later she's in the semis of the U.S. Open and then the next year she wins three Slams. It was instant. She came out of nowhere and won everything."

Back then, Hingis was a force of nature on and off the court, making waves with her dynamic game and her controversial remarks about the Williams Sisters and Amelie Mauresmo.

"She was so unapologetically confident," Neppl said. "There was no filter on her mouth."

Perhaps it was all too much, too soon. Hingis faded from the top nearly as quickly as she arrived: the Swiss player won the last of her five singles majors at the 1999 Australian Open, and three years later, after 209 weeks atop the singles rankings and 35 weeks atop the doubles rankings, she retired from the tour for the first time. She had a brief comeback in 2005, and was able to replicate some of her previous success, but she was a non-factor at the majors and ended up retiring again in 2007 after testing positive for cocaine. While she denied taking the drug, she said that she didn't want to fight the doping officials.

Six years after that, just days after being inducted into the Hall of Fame, Hingis announced she was returning once again. Singles was too much for her to handle in her mid-30s and after such a long absence from the tour, so she relegated herself to doubles only. With that, Hingis -- who last held the No. 1 doubles ranking in 2000 -- set out to be the best doubles player in the world once again.

Mirza's path was both less disjointed and less decorated, but the overall arc was similar. The best Indian female tennis player in history had early success as a teenager, skyrocketing up to No. 27 in the singles rankings; she became first South Asian woman to be appointed as a Goodwill Ambassador. Incessant wrist problems bothered her for years, though, and finally, in 2012, Mirza decided to focus solely on doubles, where she truly believed she could be the best.

With their singles days behind them, and success and fame already in hand, both Hingis and Mirza were able to carry a compatible self-confidence and passion into their partnership.

"I always believed," Hingis said after the duo won Wimbledon, her first women's doubles title in 12 years. "Without that you can't come out here and play and compete at this level. I always felt like I had one of the best volleys in the world, one of the best backhands in the world, so you got to believe in something if you want to win."

If tennis is a game of inches, doubles is a game of millimeters. There is no margin of error, particularly outside of the majors thanks to the WTA's no-ad scoring system and 10-point deciding set super tiebreaker in doubles. That's part of the reason why upsets are so common, and why the dominance that #SanTina has established is practically unheard of in today's game.

"Doubles points are so quick. One mistake, you get behind and boom. It's so hard to come back," Neppl said.

Hingis and Mirza have rarely found themselves behind, particularly during their current 31-match winning streak. When they do get in a hole or have a bad day, they've mastered another crucial component of doubles: picking each other up.

Doubles is as much about trust and forgiveness as it is forehands and volleys, reflexes and rhythms. The fulcrum of the game is the relationship between partners. It doesn't matter if Mirza's forehand is a perfect complement to Hingis's backhand if the two are never on the same page.

Luckily, they have that part covered, too.

"We react to situations we know," Mirza said after the two won the WTA Finals. "Like I know how [Hingis] reacts to morning, and she knows how I react to a situation when I'm not feeling great. I think when you know that about each other you can help each other out. That's what doubles is about, and any sport where you have two people. It's the communication. We leave it out there. We try and be honest as we can with each other how we're feeling."

With their congruent goals, compatible games, and communication skills, it's certainly easy to see why Hingis and Mirza have become the pair to beat. Unfortunately for competitors, they're determined to keep it that way.

"It's not like because we've been winning so much we take it for granted, really. We try every time. We're the hunted," Hingis said after the duo won their 11th title. "Everyone plays their best against us, and we have to keep playing our best as well to win."

So far, so good.

Correction: An earlier version of this story incorrectly stated that Hingis has won four mixed doubles majors since returning to the tour in 2013; she has won three.