NEW YORK -- The ball, struck almost timidly, made it about halfway up the net before dropping to the blue court.

There was a collective, involuntary gasp from the sellout crowd on Arthur Ashe Stadium, for this was a supreme surprise.

Because Serena Williams, of all people, isn't supposed to double-fault on break point -- even if it was against her sister Venus in a match with epic, historic consequences.

Serena lost that game and, eventually, the second set. And then, for the 11th time this year, she found herself in a winner-takes-all third set at a Grand Slam. And, as the world now knows, she ran away with it Tuesday night, beating Venus 6-2, 1-6, 6-3.

The No. 1-ranked Serena has won all 26 of her major single matches so far this year and finds herself only two victories away from the elusive calendar Grand Slam. She'll play Italian Roberta Vinci on Friday for a place in the Saturday final.

There's more than anecdotal evidence to explain why Serena Williams succeeds in three-set matches. Streeter Lecka/Getty Images North America

In retrospect, that 68 mph double fault actually underlined just how clutch Serena Williams is.

"She has of course a wonderful mental game," Venus said afterward. "But she also has ability to come up with a great shot when she needs it.

"It's been the hallmark of her career."

Exactly. Think of Michael Jordan or Peyton Manning or David Ortiz doing what they do. We imagine our favorite athletes performing at their best when, as Venus says, they need it most.

This, of course, goes against human nature. When the stakes are great, throats constrict, muscles contract and nerves suddenly appear. It's a medical fact, and it makes rising to the difficult occasion a very difficult trick, indeed.

Over the years, we have seen Roger Federer and Rafael Nadal respond exceedingly well to that pressure of the moment. Serena, too. But there has never been anything more than anecdotal evidence they really raise their game, a precise metric that proved it.

Until now.

The numbers, as we shall see, suggest Serena is the consummate clutch player.

And, perhaps more important, she is lights-out in the third set.

Serena Williams' 2015 Grand Slam statistics Serena Williams has played 11 three-set matches in the majors this year. Here's how her third-set stats meaure up. Category Overall 3rd set First-serve win pct. 73 77 Second-serve win pct. 47 53 Ace pct. 13 18 Receiving points won pct. 46 52 Break-point percent 41 47

Qualifying and quantifying 'clutch'

Meet Benjamin Tom.

He's a complicated fellow, who played on Cal-Berkeley's national championship club tennis team. He also graduated with a degree in Business Administration -- and in Molecular & Cellular Biology. He was wafting along at the Georgetown University School of Medicine when he suddenly wondered if he really wanted to be a doctor. He asked the school's administration for a leave of absence, which was granted. Now, he's pursuing his dream -- sports analytics.

His parents, naturally, are very proud.

Tom happened to be coaching ESPN analyst Pam Shriver's son, George, on a Southern California under-10 team, when the two got to talking.

"Pam and I were brainstorming," Tom explained, "about how to look at the numbers differently in tennis."

He was aware of how some analytics people quantified NBA clutch play, measuring a player's contributions in the last two minutes of a game and when the score differential was five or less. Could the two come up with a similar model for tennis?

Here is what Tom and Shriver settled on for defining clutch moments in tennis:

• All break points

• All set points

• All tiebreaker points

• All deuce or advantage points when a total of eight or more games have been played, including 30-all and 40-30.

• All points played in the deciding set, when the game differential is no greater than three -- the third set for women and the deciding set for men, either three or five.

So while you've been enjoying Serena's march through the Grand Slams, Tom has been busy breaking down that trek -- in excruciating detail.

And here's what he's discovered:

In winning all seven matches at Wimbledon, Serena outperformed the field in 9 of 10 statistically comparable categories, many by a substantial margin. While the other 127 players average 67 percent of first-serve points won, Serena won 80.3 percent. The field averages winners 15.3 percent of the time, while Serena was at a robust 24.4. Her edge in ace percentage was 18.3 to 5.

But that is reasonably predictable stuff.

Tom also calculated how Serena did measured against herself, comparing her total points to "clutch" points. Of the 962 points she played at the All England Club, 695 were defined as non-clutch points, with 267 deemed to be "clutch."

Serena's already extraordinary play elevated to a higher level in the crucible of clutch. She was better in 8 of 10 categories, including dramatic improvements in winners versus unforced errors, first-serve percentage and aces.

Tom has also crunched the numbers here at the US Open, and the pressure of chasing a calendar-year Grand Slam seems to have taken a toll. Taking into consideration her past nine major matches -- the last four rounds of Wimbledon and the first five in New York -- her clutch statistics are down ever so slightly.

Even so, Serena is still outperforming the field by a wide margin. It's worth noting that her level of play is so high that she has been winning matches in majors playing less than her best. This was especially true at the French Open, where she won in straight sets only twice -- and dropped the opening set no fewer than four times. The average score in those ultimate sets: a resounding 6-2.

Which brings us to Serena's ace, as it were, in the hole.

Three and out

Twenty-seven years ago, when Steffi Graf last achieved the calendar-year Grand Slam, she lost only two sets in 27 matches -- one to Martina Navratilova in the Wimbledon final, the other to Gabriela Sabatini in the US Open final.

Graf won the deciding set in each case by a 6-1 score.

Serena, who seems to thrive (not merely survive) on drama, has escaped each of the 11 times she's put herself up against a one-set-all wall.

Looking at Serena's 26 matches Grand Slam matches, Tom isolated her third-set statistics versus the overall stats for all those matches. As you might expect, Serena plays markedly better in the third set.

Serena surpasses herself in 7 of 8 categories and is even (60 percent) in first-serve percentage. Of particular note, the ratio of winners to unforced errors (which goes from 1.4 to 1.67) and percentage of total points won (56 to 60) improve appreciably.

"It would be the equivalent of having an NBA or major league player, who already leads the league in nearly every statistic, get even better if we only measure their stats in overtime or extra innings," Tom said.

And then there is the ultimate comparison -- the third-set numbers versus the overall numbers of those 11 three-set matches that have delivered Serena to the precipice of history.

Across the board, the margins increase and, in some cases, double or more.

The critical category of break-point conversions, for instance, improves from a 1 percent increase to 6 percent. Likewise, second-serve winning percentage, receiving points won and winner/unforced error ratios all soared when it mattered most.

"We have always felt that Serena has been the greatest of clutch players," Shriver concluded. "With Ben Tom's analytics, we can put more value and context to her amazing levels under at the biggest stages, especially in final sets."