Roberta Vinci’s upset of Serena Williams in the U.S. Open semifinals last year sent shock waves through tennis by slamming the door on Williams’ Grand Slam bid.

It also catapulted Vinci, then ranked No. 43 and better known for doubles success, into one of the most unusual Grand Slam finals of the Open era.

Vinci, then 32, faced her Italian countrywoman and longtime friend Flavia Pennetta, a 33-year-old seeded 26th. Each was playing in her first Grand Slam singles final.

Pennetta won and announced after the match that she would retire at the end of the season.

Vinci has since risen to No. 8, but it may take another magical run at a major for her to avoid ending up like other unlikely Grand Slam finalists, including Sharon Walsh (1979 Australian Open), Steve Denton (1981 and 1982 Australian Open), Mikael Pernfors (1986 French Open) and Martin Verkerk (2003 French Open).

Those players’ breakout Grand Slam performances were not, for the most part, backed up by subsequent success.

“It was lucky,” said Vinci, a five-time Grand Slam champion in doubles. “I took my chance, and I played a great U.S. Open.”

Vinci has failed to advance past the third round at any of this season’s three majors, which is in line with her career results. The best she had done at a major before last year’s final was back-to-back quarterfinal appearances in New York in 2012 and 2013.

How did those other spoilers defy logic and expectation?

Some got in a zone. Some benefited from soft draws. Some, like Vinci, possessed unorthodox games that were perfect for disrupting heavily favored opponents.

All seized the moment.

“You have to always fight, be positive, and anything can happen,” said Vinci, who had told her travel agent to book a return flight to Italy the day before she played Williams in September.

Verkerk had never won a match at any of the four major tournaments when he nearly ran the table 13 years ago in Paris. Ranked 46th, he knocked off four seeded players and finished as the runner-up.

“They call it the flow,” Verkerk, now 37, said of his out-of-this-world form. “You start hitting balls like they are watermelons. You cannot miss.”

Verkerk did not advance past the third round at a major again. He retired in 2008 with two ATP Tour titles and a career record of 59-70.

Like Verkerk, Pernfors and Denton were winless at majors and unseeded when they reached a Grand Slam final.

Both were college standouts. Pernfors had won back-to-back NCAA singles titles at Georgia in 1984 and 1985. Denton had been an All-American at Texas in the 1970s.

Neither had shown world-beating potential as a pro.

Playing on grass at Kooyong Stadium in Melbourne, Australia, in 1981, Denton eked out four five-set victories, a record.

“When you’ve gotten out of jail so many times, you kind of feel almost a little bit invincible,” said Denton, 59, now the men’s tennis coach at Texas A&M.

Pernfors, a wily baseliner who liked to draw opponents into the net and jerk them around with angles, beat four seeded players in 1986, including No. 5 Stefan Edberg and No. 3 Boris Becker, before losing to Ivan Lendl in straight sets.

“I certainly didn’t see myself as someone who was going to make it to the final of the French Open,” said Pernfors, who did not blossom until he left his native Sweden to attend college in the United States. “I pretty much just went there hoping to win matches.”

These surprise finalists carried few expectations before their big runs but were brimming with confidence after playing well in the weeks before.

“I was just on such a tennis high those two weeks — the feeling that I could play with anybody,” said Pernfors, 53, a regular on the seniors’ tour.

Each of the spoilers survived close calls or mentally overcame a difficult opponent early in the tournament.

In her opening match a year ago, Vinci trailed Vania King 4-2 in both sets but rallied to win, 6-4, 6-4.

In 1979, Walsh escaped a three-set opening tussle with net-charging Marcella Mesker. Walsh did not drop a set again until the final, which she lost to her fellow American Barbara Jordan, who was seeded fifth.

In the second round in 2003, Verkerk was down two sets to one and 5-2, 40-0 to Luis Horna, who had upset fifth-seeded Roger Federer the round before. Verkerk saved all three match points with booming serves.

“I served and volleyed three times at love-40 to go shake his hand,” Verkerk said jokingly.

Pernfors was assisted by momentum-slowing rain during a five-set comeback win against Edberg in the second round.

Denton said he had never beaten Kevin Curren, his Texas Longhorns teammate and doubles partner, at any level when they clashed in the second round of the Australian Open 35 years ago. When Denton lost the first two sets, he figured he was done.

“Then I just got on a roll,” Denton said.

Favorable draws, depleted by upsets or circumstance, helped.

Because the Australian Open often overlapped with the December holiday season and was considered less prestigious, many top players skipped the event in the first 15 years of the Open era, which began in 1968.

The highest seed that Denton beat in 1981 before losing to Johan Kriek in the final was No. 9 Shlomo Glickstein.

Virginia Ruzici, who finished No. 13 in the WTA rankings in 1979, was the top seed the year Walsh reached the final in Melbourne. Walsh was seeded fourth.

“When it’s a full draw and everybody was playing, it was a lot tougher,” said Walsh, who never won a singles title and whose ranking peaked at No. 22.

Vinci caught some breaks, too. She received a walkover in the fourth round against Eugenie Bouchard, a former Wimbledon runner-up. Vinci’s highest-ranked opponent before she toppled the top-ranked Williams was No. 40 Kristina Mladenovic.

Vinci was no upstart. She had big-match seasoning in doubles and owned nine WTA singles titles entering last year’s Open. Her game of slices and angles rattled Williams.

Once in the championship match, none of these surprise finalists came close to winning. Vinci, Verkerk, Denton, Pernfors and Walsh went a combined 1-16 in sets.

Verkerk, who opened a tennis academy in the Netherlands after pulling away from the sport for many years, called the 2003 French Open the best experience of his turbulent athletic life.

Denton never won an ATP-level title and finished his career with a losing record (108-116). But he did manage to reach the singles final in Australia again in 1982, which makes him an exception among these far-fetched finalists. Seeded No. 2, he lost in straight sets to Kriek in a rematch of the 1981 final.

Pernfors’ lack of subsequent success at the French Open is particularly striking.

In four attempts over the eight years after his appearance in the 1986 final, he never won another match at Roland Garros.

“I can’t explain it,” Pernfors said with a chuckle. “I still tell people I have a winning record there.”

He was 6-5.

But, these unlikely finalists said, the lasting effects were positive. Those appearances landed them endorsement contracts and affirmed their hard work. The major runs also provided new motivation, even if some of the players — Denton, Pernfors and Verkerk — were later hampered by injuries.

All things considered, they deserved, and they earned, their place in history.

“You have to be proud, no matter who is in the draw,” said Walsh, 64.

The success generated larger expectations, but no one regrets it. And it sent a message to their peers.

“If you get a break or two,” Denton said, “you are closer than you think to being really successful.”