Two more wins.

That's all Serena Williams needs to complete the Grand Slam, a feat so rare that it has been accomplished just six times in tennis history.

Steffi Graf was the last winner, taking all four major tournaments in 1988.

Williams has already won four straight major titles, dating back to last year's U.S. Open, but she has never won all four in the same year.

"I don't know why it took me so long," Williams said after winning her quarterfinal match Wednesday night.

Williams would be the fourth woman to win the Grand Slam, joining Graf, Maureen Connolly and Margaret Court. Martina Navratilova once won six straight Grand Slam tournaments, but never won all four in the same year.

On the men's side, Don Budge also won six straight, while Rod Laver twice completed the calendar-year Grand Slam but never won more than four in a row.

Here's a look at some rare accomplishments in sports:

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BASEBALL:

Throwing a perfect game is the pinnacle of pitching and hitting .400 in a season is the benchmark for batting, but neither are the rarest feats in baseball.

Major league pitchers have thrown 23 perfect games and 27 hitters have finished with .400 averages — none since Ted Williams in 1941.

Compare that to hitting two grand slams in one game, something that has happened 13 times, the last coming in 2009 by Josh Willingham of the Washington Nationals. Or an unassisted triple play, which has only happened 15 times.

Seventeen hitters have won the triple crown — leading the league in average, home runs and RBI — including Miguel Cabrera in 2012. Williams and Rogers Hornsby did it twice.

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BASKETBALL:

In the NBA, there is one mark that stands above all others: Wilt Chamberlain's 100 points in a game set in 1962.

The closest anyone has come to matching that was Kobe Bryant in 2006 when the Los Angeles Lakers star scored 81 points.

Chamberlain's feat was no fluke. He holds six places in the top 10 scoring list, and 15 in the top 20.

Another rarity in basketball is the quadruple-double. Only four players have reached double-digits in four categories in a game: David Robinson, Hakeem Olajuwon, Alvin Robertson and Nate Thurmond.

Robinson, Olajuwon and Thurmond all did it in points, assists, rebounds and blocked shots, while Robertson did it with steals instead of blocks.

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FOOTBALL:

Only seven times has a player rushed for more than 2,000 yards in a season — the last coming in 2012 when Adrian Peterson ran for 2,097 yards.

And only one player, O.J. Simpson, managed to run for 2,000 yards while playing 14 games. All the others played in 16 games.

The 1972 Miami Dolphins remain the only pro team with an undefeated season, winning all 17 games they played.

In college football, Archie Griffin stands alone as the only player to win the Heisman Trophy twice.

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HOCKEY:

Goal-scoring goaltenders — now that's a rarity.

Only 11 goalies have scored in NHL games, the first coming in 1979 when Billy Smith of the New York Islanders was given credit for a goal after the puck when into the net off an opposing player.

The most recent came from Mike Smith of the Phoenix Coyotes in 2013 from a shot on goal.

Soccer goalkeepers have a much higher ratio of scoring. Rogerio Ceni of Brazil is credited with 130 goals in his career — mainly from penalties and free kicks.

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GOLF:

The rarest shot in golf? The almost-mythical condor.

That's when a player scores a 4 under on one hole, essentially a hole-in-one on a par-5.

There have supposedly been at least four condors in history, though none in official tour tournaments or on professionally accredited courses.

As for golf's version of the Grand Slam, winning all four majors in one season, only Bobby Jones in 1930 has done it. Back then, the four biggest tournaments were the British Open, U.S. Open, British Amateur and U.S. Amateur.

Tiger Woods did win four modern majors in a row, but not all in the same year. He won the U.S. Open, the British Open and the PGA Championship in 2000 and then the Masters in 2001.

No woman has won four majors in one year, but Babe Didrikson Zaharias won all three majors played in 1950.

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HORSE RACING:

The triple crown in horse racing has only been accomplished 13 times, including this year when American Pharoah won the Kentucky Derby, Preakness Stakes and Belmont Stakes.

The previous triple crown came in 1978, when Affirmed matched the feat achieved by Seattle Slew a year earlier.