Before her match against Wendy Turnbull in the semifinals of the $100,000 Virginia Slims of Washington tennis tournament yesterday, Chris Evert said they had played "once before, a couple of years ago on clay," but that she couldn't recall much else.

No wonder. Evert has won 21 consecutive tournaments and 101 straight matches on her favorite surface, dating back to August, 1973. Presumably, her earlier meeting with Turnbull didn't last long.

Turnbull, 24, an Australian who likes to play attacking tennis, had hoped to do better on the fast synthetic carpet at Capitol Centre. But she could manage only four games, Evert winning, 6-3, 6-1, in 49 minutes.

Evert, eeeded No. 1 here, thus moved into the final. She will go after the $20,000 first prize at noon today.

Her opponent will be third-seeded Martina Navratilova, who beat second-seede Virginia Wade, 7-6, 6-0.

If she wins, Evert will be the first player in the six years of this Slims event to repeat as champion. She beat Virginia Wade in last year's final, succeeding Nancy Richey (1972) Margaret Court (1973), Billie Jean King (1974), and Martina Navratilova (1975).

Turnbull did not seen awed by the switch from George Washington Unoverstiy's intimate Smith Center, where the tounament was played through Friday's quarterfinals, to cavernous Capital Centre, which was darkened, like a theater, except for the lights focused on the court.

At 5-foot-3, 120 pounds she is one of the quickest players in women's tennis but inexperienced against top competition. She played about as well as she could at the start, but was clearly overmatched.

Turnbull's biggest wins to date - the 1976 Austrian and Japan opens, which had weak women's fields - hardly compare to Evert's laurels, which read like a What's What of tennis: two Wimbledon, two U. S. Open, three Virginia Slims, two French, two Italian championships and more.

Evert was not nearly as sharp as she had been in annihilating fifth-seeded Sue Barker, 6-1, 6-2, Friday. She made some loose ground-stroke errors, especially off the forehand, but had almost no pressure exerted on her serve.

Evert lost only seven points in her eight service games. She never had a break point against her, and served three love games, even though she got only 25 of 39 first serves, compared to 28 of 57 for Turnbull.

The natch was never a contest, even though turnbull grittly hung in from the start. She banged an ace to hold serve for 3-4, having played three nice points from 0-30, but Evert held her serve at love in the sixth and eighth games and broke Turnbull at 15 for the set in the ninth. The Australian badly netted a backhand volley on the set point.

Turnbull held her serve for 1-1 in the second set, but by this time her groundstrokes were getting a little ragged. She wa underhitting he slice backhand, overhitting her forehand and obviously feeling the relentless pressure of Evert's steady baseline assault.

Evert broke for 3-1 after Turnbull had come back from 0-40 to have one game point. On the last two points of that game, Turnbull missed forehands she tried to steer across court although the near side of the court was wide open.

Turnbull started to look weary, through the fatigue was probably more mental than physical because she did make a few superb "gets," including a memorable one on an Evert drop host that looked like a sure winner.

Evert broke again for a 5-1 lead, forcing a backhand error with a two-fisted backhand off a Turnbull drop shot that didn't drop.

Evert then served cut the match at love.

Turnbull had the right game plan for playing Evert. She mixed up her shots, served and volleyed when Evert didn't expect it and won a few points on her serve that way, and tried to come in behind her returns of Evert's second serve.

But she could not execute her shots well enough to make the game plan come any closer toworking than the first time they played.