A 33-year-old who grew up in California and a 28-year-old from Serbia -- both at the prodigious peak of their powers -- dominated tennis in 2015.
Serena Williams and Novak Djokovic each won three of the four Grand Slam singles titles, a once-in-a-generation phenomenon that had only occurred twice in the past 46 years. Steffi Graf and Mats Wilander took seven of eight in 1988, as did Aussies Rod Laver and Margaret Court in 1969.
"I think I knew that," Wilander recently told ESPN.com from his Idaho home. "Steffi won all four and I won three. There's your answer. Yeah, it's been awhile since we've seen that kind of dominance on both sides of the game. It was fun to watch."
All season long, through to the second-to-last major match of the season, tennis fans were captivated by Serena's Slam attempt. But in retrospect, did Djokovic -- chronically under the radar, typically upstaged -- actually have the better season?
It's a cruelly subjective question, one that weighs the entire body of work versus degree of difficulty, epic quantity and excellence against a narrower slice of dazzling quality.
The bare numbers favor Djokovic:
• Williams was 53-3 (.946) and won five titles. That winning percentage is the women's 12th-best in the 48 years of the Open era.
• Djokovic was 82-6 (.932) and won 11 titles, including a record six ATP World Tour Masters 1000 events. That's the sixth-best winning percentage for men in the Open era.
Djokovic played against arguably better competition -- and far more often. His record against top-10 players was 31-5, compared to 6-1 for Serena. Djokovic reached all four major finals and produced a Grand Slam singles record of 27-1 -- the lone loss coming at the hands of Stan Wawrinka at Roland Garros. Djokovic played in eight of the ATP's nine Masters events, skipping Madrid and losing to Andy Murray and Roger Federer in the finals of Montreal and Cincinnati, respectively.
Williams, the WTA's Player of the Year for the seventh time, lost in the semifinals of the US Open and finished 26-1 in majors. She finished as the No. 1-ranked player for the fifth time -- and the third year in a row. Serena played in six of the WTA's nine Premier 5/Premier Mandatory events, missing Dubai, Wuhan and Beijing. She won two (Miami and Cincinnati), lost two (to Petra Kvitova and Belinda Bencic in Madrid and Toronto) and granted walkovers to her opponents in Indian Wells and Rome, two of her three for the season. Djokovic had no walkovers.
However, Williams won the season's first three majors -- the Australian Open, French Open and Wimbledon -- and her pursuit of the fourth, in New York, drew even mainstream, less-than-casual tennis fans into the gripping drama. She was, notably, Sports Illustrated's choice for Sportsperson of the Year.
Perhaps more impressive: Williams did it at the remarkable age of 33. Will Djokovic be capable of winning three Slams in a season five years from now?
Another area to consider: margin of victory. Only three of Djokovic's 28 major matches went the five-set distance. Meanwhile, 12 of Serena's 27 major matches required the women's maximum three sets. Overall, 19 of her 56 matches went three, while Djokovic was pushed to the three- or five-set limit 18 times in 88 matches.
Djokovic finished with a flourish, winning 27 of his final 28 matches. He became the first tennis player to earn more than $20 million in prize money in a single season.
After losing in the quarterfinals to Ivo Karlovic in his opening tournament in Doha, Djokovic made the finals of the last 15 events he played. Serena, suffering from injuries, shut it down after the US Open.
Here's how the experts broke down Williams' and Djokovic's 2015 seasons:
Darren Cahill, ESPN analyst and former coach of Lleyton Hewitt and Andre Agassi
"Any player that can win three majors in a season and only lose a handful of matches has recorded a remarkable achievement, male or female.
"Novak, Rafa and Roger have all accomplished the feat in recent years, but no man since Rod Laver in 1969 has won the first three majors in a season. Serena is the first female to accomplish it since Steffi Graf in 1988.
"Serena brought a lot of extra attention to tennis in 2015 and the sport is better for it. The men have enjoyed a golden era with Rafa, Roger, Novak and Andy, and have quite rightfully owned the majority of the headlines over the years, but this year belongs to Serena and the WTA.
"From the moment Serena won the French Open, she delivered tennis and the possibility of a Grand Slam at the forefront of any sporting conversation. The casual tennis fan became an avid fan following the progress, and the non-tennis fan even tuned in to see if history would be made. With that came expectation, pressure, nerves, excitement, opportunity, obligations and many other factors that are tough to quantify."
Pam Shriver, ESPN analyst and 21-time Grand Slam doubles champion
"Well, the big difference is that six years in between them. It's way different being in your mid-30s and late-20s. I would just want to add that as a caveat, the competition that Novak is facing probably raises him above. Even Federer's greatest year  came before Rafa and Novak really, really came into their primes. It almost equalizes it out. That's the difficulty in this discussion."
Brad Gilbert, ESPN analyst and former coach of Andre Agassi, Andy Roddick and Andy Murray
"It's a good question. Serena had the crescendo. She was facing the basket, going for the Slam. Novak didn't get that look. But his overall body of work for the year is above hers. She got the look, he didn't. His year is better, but ask him if he'd probably rather have the look she did.
"That look at a Grand Slam is something he's maybe never going to get. Serena's was the first real look since Steffi [Graf, in 1988]. The last guy was Rod Laver, [in 1969]."
Mary Carillo, Tennis Channel and NBC analyst
"Novak. Oh, yeah. Obviously, Serena played with a lot more pressure in the US Open than Novak did -- and Wimbledon, too. The quality of the player, the level of excellence he shows -- apart from Wawrinka at French -- you give him the season.
"Serena had a very snappy little season. I just think Novak played better -- and against better opponents, day in day out, major after major. Novak had the better season, Serena had the bigger year. She was three sets from a calendar Slam. She dominated the headlines, and for a very good reason. But then she stopped playing. Novak, whose dreams were over in June, kept on going with a full fall."
Toni Nadal, coach and mentor of Rafael Nadal
"I think Djokovic, but Serena had a great year, too, as she also won three Grand Slams. Maybe Djokovic, as he made the final of the other one [Roland Garros], and Serena lost in the semifinal of the one she didn't win [the US Open]. Sometimes this season, it was impossible for anyone to beat Djokovic. With Serena, it was a little less impossible for her opponents."
Mats Wilander, seven-time Grand Slam singles champion
"Serena played great, but she wasn't as dominant as Steffi [in 1988]. She won the French Open final [over Natasha Zvereva, 6-0. 6-0] in 32 minutes. I know because I was the next match on. I think you have to account for who they're beating in the finals.
"To me, Djokovic's year is obviously much better. I mean, you would have to say it's almost unbelievable when he dominates that much. To win three twice -- suddenly you have to say, 'Hold on, maybe the opposition isn't missing because he's beating the best guys. He's so good, day in and day out, on all the different surfaces. With that style, you can't beat him.
"To me, for those two players, it's still disappointing they didn't win all four. Those were two of the most disappointing matches -- in the biggest matches of the whole year. Vinci was the biggest upset of the year, and right behind was Djokovic-Wawrinka. In retrospect, that's the scary point: They both should have won the calendar Slam."
Todd Martin, CEO of the International Tennis Hall of Fame
"Serena's head and shoulders above the rest. The age thing? The only way I could listen to an age element in this discussion would be the fact that as you get older, it's harder to find the sheer motivation to do all the preparation. She's obviously doing better preparation than she has in the past. That's a credit to her and her coach, [Patrick Mouratoglou].
"I would have loved to have seen Novak handle New York in light of a French Open win. When you're talking about people who have won 10 Grand Slams, they are going to get excited toward the end of Slams. You don't see those people hitting the wall mentally at the finish line. They've been there, they've done that and they trust themselves to figure it out."
"Going 27-1 is not quite the same as being 26-0 entering the semifinals of the last major."
Paul Annacone, Tennis Channel analyst and former coach of Pete Sampras and Roger Federer
"Serena's season had more pressure, because she went into the last one with her hopes still alive. It's a little bit apples and oranges. There's never not pressure for Serena's dream to be alive, so that's a whole different kettle of fish. Novak's dream was dead at US Open, not boiling over with pressure, just simmering.
"Athletes are staying great longer. I've always felt that the great athletes are an aberration -- they're just different. Serena, to do something spectacular like that, is not so surprising. But you have to be smart to do that, and make some good decisions. She's doing things off court to be fully prepped on it. Her diversity in the things she liked and her ability to separate from the rigors of the tour is allowing her to play this long. It's a good thing for her, and for us tennis fans, too."
"Obviously, Serena's pressure was building and building. Novak's year, on paper, is one of the best of all time. It's tough to compare because the scenarios are different.
"Look at Novak's body of work in 2015. He was actually picking up steam at the end. It's just incredibly special the guy is one herculean performance from winning the Grand Slam when men's tennis has never been stronger."