The 2013 women’s tennis season can be summed up in two shots, hit in the same game, by the same player, in Istanbul on Sunday. Serena Williams, who had lost the first set to Li Na, was serving for the second at 5-3. Li had played some of the best tennis of her career up to that stage, and she wasn’t quite ready to cave to the world No. 1. Roping returns, rolling her backhand for winners, and exhorting herself more forcefully than normal, Li reached break point twice. She was playing well enough to make you imagine, if she had won one of those points, that she might have found the nerve to pull off the upset. But, of course, she didn’t win those points. Serena won them, by belting two forehands past her. She won them by not giving Li a chance to play the way she had been playing. Serena wouldn’t lose another game.

As those shots flew by her, Li became the 78th and last woman to lose to Serena in 2013. She also became the latest player to challenge Serena on a big stage for two sets, only to have that challenge rendered utterly futile in the third. With her 2-6, 6-3, 6-0 win, Williams finished 5-0 at the WTA Championships for a second straight season. At age 32, she completed what may have been the finest year of an already very fine career. Serena set personal highs by going 78-4 and winning 11 tournaments; that’s one-sixth of all the titles that she has won over the course of her 15-year career. Among them were the French Open and the U.S. Open, her 16th and 17th major championships. Just as impressive was Serena’s 14-2 record against opponents in the Top 5. The best kept trying, and failing, to beat her.

I don’t know whether Serena’s 2013 is better than her 2002, a year in which she won three Grand Slams. But what seems unique about this season is the way she showed vulnerability on a regular basis, yet ended up so thoroughly sidelining her competition. She made it look tough before she made it look easy, and, in one of those proverbial “signs of a champion,” she won even on her worst days.

In April, Maria Sharapova was up a set and a break on a sluggish Serena in the Miami final; it looked like this was the day when Maria would finally end her decade-long run of abject defeats at the hands of her nemesis. Not quite; Serena won 6-0 in the third. Two months later, Sharapova played a very good match against Serena in the French Open final, only to lose 6-4, 6-4. Since that day, Maria’s record is 1-2.

The next challenge came from Victoria Azarenka. After beating Serena in Cincinnati in August, she faced her three weeks later in the U.S. Open final. Despite battling back from a set and two breaks down, Vika watched helplessly as Williams gathered herself, stopped making mistakes, and won the third set 6-1. Since that day, Azarenka’s record is 1-5.

Yes, Maria and Vika have both had their non-Serena-related problems—injuries, coaching issues, lack of motivation—over the last five months. But there must also come a time when trying and failing to chase down the world’s best gets old. With Maria absent and Vika barely present in Istanbul, the task fell to Li. She became the latest to be granted a glimmer of hope, only to have it snuffed out. We’ll see how Li reacts. This season, and especially this tournament, has been a positive. At 31, she reached a career-high ranking of No. 3 and made her first WTA Championships final. With coach Carlos Rodriguez prodding her, she has had the most consistent six months of her career, and has even taken the daring late-career step to begin serving and volleying. Now she just needs to learn when to serve and volley. Yesterday, at 3-3, 0-15 in the second set, with the match in the balance, Li followed her serve in and sailed a forehand volley long. It doesn’t take hindsight to realize that wasn’t the time to go to the experimental well.

Is Li the woman to conquer Serena in 2014? I doubt it. Whatever innovations are in her future, and however much upbeat energy she and Rodriguez can muster, she’s still past 30. And while Li fought Williams most of the way on Sunday, she didn’t fight her all the way. Li followed up her ill-timed net approach at 3-4 with a double fault to make the score 0-40. She believed in her chances, until she didn't believe in them.

That was the theme of Istanbul: Is there a woman who can stand up to Serena for two whole sets? Serena said she hit the “wall” physically before her semifinal with Jelena Jankovic. Her 80 matches in 2013, the most of her career, had caught up with her, she said. I believe her, though I wonder if her sluggishness was more a product of being 32 years old and playing a fourth match in five days. Daily recovery, veteran stars say, is the most difficult aspect of playing past 30, and even Serena must feel the effects of age. Either way, it wasn’t the first time we’ve seen her look exhausted and unhappy and still come away a winner. Jankovic saw a pattern, and called Serena’s on-court demeanor bad sportsmanship. I wouldn’t go that far, but it is difficult to play someone who hardly seems to be trying, yet is still doing enough to stay in a match; it was wearying enough just watching Serena that day. In the end, though, it was Jankovic’s job to ignore what was going on across the net and find a way to win. She couldn’t do it. (Serena may have been lacking in intensity in the semis, but she made it up for it in the final. She threw a season’s worth of shrieks and death stares in Li’s direction.)

Vulnerable but unbeatable, dramatic but inevitable, Serena finished her most productive year in appropriate style this weekend. Maria, Vika, Li, Sloane Stephens: All of them slowed Serena down for a moment; all of them ended the season getting smaller in her rearview mirror. Yesterday, watching Serena hit those two forehand winners against Li at the end of the second set, and then run away from her in the third, it became clear again why all of her challengers lose their nerve and their will when they play her. She’s better than they are.