The Washington Diplomats, reeling from their worst defeat of the season, began preparations yesterday for their final regular-season game Sunday against the Cosmos, knowing the outcome could have a major bearing on their playoff status.

Following their 4-0 loss to the Atlanta Chiefs Wednesday, the Dips have a 19-10 record, good for 169 points and second place in the NASL National Conference East behind the Cosmos. The Minnesota Kicks, 5-3 winners over Detroit Wednesday, are 20-9 for 175 points.

With their final game at home against Memphis, the Kicks are all but certain to nail down the second-best record in the National Conference, meaning that if they meet the Dips in the second round of the playoffs, they will have the home field advantage-.

The Dips must also be concerned with the Vancouver Whitecaps, 19-10 for 164 points. The Whitecaps finish their season Sunday at Seattle. If they win and the Dips lose at the Meadowlands, the Whitecaps will finish with the third-best record in the conference. That would mean that if Washington survives the first round, its probable second opponent would be the Cosmos.

The Dips still do not know who their first-round opponent will be. Currently, the Los Angeles Aztecs are two points ahead of the Dallas Tornado, both with 17-12 records for the fifth-best mark in the conference. The Aztecs close Sunday in Portland while the Tornado finishes at Houston, the Hurricane having clinched the best record in the American Conference.

The Dips will open either in Dallas or in Los Angeles, depending on which team finishes with more points. That first game will be next Wednesday, with the return match - and the ensuing minigame if the two games are split - at RFK Stadium either Aug. 18 or 19. The game will be on Sunday, the 19th, unless ABC-TV chooses it for telecast on the 18th.

The rest of the playoff picture is clearing rapidly. Seven National Conference teams have clinched spots, with Rochester, Seattle and Toronto tied with 124 points each and having one game left to play. The Lancers would clinch the spot with a nine-point win over New England Friday because they have one more victory than the other two clubs.

In the American Conference, seven of the eight spots have been clinched, with New England three points ahead of Philadelphia for the last spot.