-- It is again time to hand out our year-end awards. Today, we look at the top on-court moments of 2015, but click here to check out the rest of our list.
Top on-court storylines of the year
1. Dual dominance from Novak and Serena
It was big when they won, and bigger when they didn't. With six Slams, 16 titles and firm holds on No. 1, this season was defined by what Novak Djokovic and Serena Williams achieved -- and the few things they couldn't. Djokovic ended the season by winning his fourth straight ATP World Tour Title and nearly doubled No. 2 Andy Murray in rankings points.
2. Big Four and one more
Stan Wawrinka's run at the French Open not only halted Djokovic from winning everything, but the win gave the Swiss two Slams in two years and prompted talk of a big five in men's tennis. And as for the other four, they actually increased their collective dominance from a year ago.
Murray had his best clay season and won Davis Cup, Roger Federer was the most regular challenger to Djokovic, and even Rafael Nadal looked back on track by the ATP Tour Finals. There may or may not be a big five, but there's a big gap between them and the others.
3. Italians shake up US Open
It was supposed to be about William's quest for the Grand Slam, but two Italians tore up the script and took the tournament for themselves. Roberta Vinci stunned Williams in the semifinals to set up a final against childhood friend Flavia Pennetta, who then proceeded to win her first Grand Slam. Watching from the box was Pennetta's fiancé, Fabio Fognini, who had already sent Nadal packing in one of the biggest matches of the men's event.
4. Nadal's clay empire falls
Since his first French Open victory in 2005, Nadal had been almost untouchable on red clay, winning nine French Open titles and more than 40 tournaments in total on the surface. But not this year. The Spaniard won only two smaller clay titles, fell to Djokovic in the French Open quarterfinals and went 26-6 on clay. The question now is whether Nadal can reconquer his territory.
5. Young guns slow in arriving
Any changing of the guard in the men's game was put on hold as the top up-and-comers took steps backwards. Kei Nishikori fell from No. 5 to No. 7, Milos Raonic from No. 8 to No. 14, 2014 US Open champ Marin Cilic from No. 9 to No.13 and Grigor Dimitrov from No. 11 to No. 28.
Now between 24 and 27 years old, they're also wearing the young gun label a little thin.
If there was any movement outside the upper-echelon players, it came from the newbies. Nine teenagers won Challenger titles this season. So far, four of them have reached the top 100.
Younger players did slightly better on the WTA tour, with 24-year-old Simona Halep catapulting to No. 2 and Garbine Muguruza, 22, making the most significant stride of the season by reaching the Wimbledon final and reaching No. 3 in the world. Others like Karolina Pliskova, 23, Madison Keys, 20, Elina Svitolina, 21, and Belinda Bencic, 19, finished in the top 20.
6. Older players thriving
Meanwhile, the tour veterans have been making a fine showing. Williams is No. 1 at 34, the oldest ever to hold the top spot, while sister Venus, 35, is back in the top 10. Federer, the same age as Williams, is No. 3, and is joined in the top 10 by 33-year-old David Ferrer. The average age of the ATP top 10 is now 30 years old.
What's more, some players are even reaching career highs in their 30s -- once seen as retirement age for most tennis professionals. Among them are Feliciano Lopez, Gilles Muller, Julien Benneteau, Guillermo Garcia-Lopez, and Victor Estrella Burgos, with a whole lot looking set to follow suit.
7. No. 1 and anyone
The WTA pecking order behind Williams became even more blurry following a merry-go-around of a season. Halep played well on hard courts, but didn't fare as well on her favored clay. Maria Sharapova started quickly, but injuries derailed her season.
Muguruza played well down the stretch, reaching the semifinals of the WTA Finals but showed inconsistency during the season. Inconsistency was also the overarching theme for Petra Kvitova and Agnieszka Radwanska.
8. Hingis turns doubles queen
She won Grand Slams in her teens before twice coming back from retirement, and now Martina Hingis is once again at the top of the game -- in doubles. The 35-year-old from Switzerland had been slowly making her way up in a doubles-only comeback, but took off once she partnered with Sania Mirza toward the beginning of the season.
9. Bryans slip down ranks
Having set just almost every significant team record in doubles, Bob and Mike Bryan slipped from their perch this season, finishing ranked Nos. 4 and 3, respectively. The twins won just six titles, their lowest since 2005 and did not win a Slam for the first time since 2004. Between injuries and growing families, they couldn't get a roll going.
10. American youngsters make waves
The conversation around the American men's game began to change this season with several talented youngsters showing their potential. The top-ranked American is 30-year-old John Isner at No. 11, with the next top prospect looking like 23-year-old Jack Sock. But there's a wave building in the junior ranks.
Three straight junior Slams were won by teenaged Americans -- Tommy Paul at the French Open, Reilly Opelka at Wimbledon and Taylor Fritz at the US Open -- joining Noah Rubin at Wimbledon a year ago. Three teenage Americans have climbed into the top 200 -- Jared Donaldson, Fritz and Frances Tiafoe. At one point this season, three of the top four junior boys were Americans.
The Slam drought is still on, as is the lack of American men in the top 10, but the turnaround might have begun.