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SAIGON, South Vietnam, April 10--Communist guerrillas faded into hiding south of Saigon tonight after killing more than 50 Vietnamese soldiers and a U.S. pilot in two days of bloody attacks. Three other Americans were wounded in fighting that brought the war to this capital's doorstep.

The second and most violent conflict swirled Thursday around the prosperous farming center of Mo Cay, 50 miles south of Saigon. That was where the American casualties occured.

At the height of the conflict, Communist ground fire brought down a Vietnamese plane, killing the U.S. pilot and his Vietnamese observer.

The pilot was identified in Washington as Capt. Robert N. Brumet, whose wife lives in Bethesda, Md. He was the 123rd American killed in action in the guerrilla war since 1962. His body was recovered.

Communist sharpshooters also winged two U.S. Army helicopters, forcing them to land. One U.S. crewman was injured slightly. Two U.S. Army men with the ground forces were wounded.

The Communist attacks were a challenge to Premier Maj. Gen. Nguyen Thanh, who has called that area one of the most critical in South Viet Nam. He has been trying to assert Vietnamese control there, where guerrilla attacks have been almost constant in recent months.