Behind veteran Vijay Amritraj and Ramesh Krishnan, India took a commanding 2-0 lead over defending champion Australia yesterday in their Davis Cup semifinal in Sydney.

India, which never has won the prestigious international team trophy, can reach the final by winning either the doubles today or one of two singles Sunday.

Amritraj, 33, who first played in the Davis Cup 18 years ago, defeated Wally Masur, 1-6, 6-3, 12-10, 6-4, after Krishnan, 26, downed John Fitzgerald, 6-1, 6-2, 3-6, 8-6.

The winner of this best-of-five series will play the winner of the semifinal between Sweden and Spain in the final in December.

Australia suffered a blow when Wimbledon champion Pat Cash withdrew from the Davis Cup team, citing an injured knee . . .

Katrina Adams of Chicago defeated sixth-seeded Tammy Whittington of Gainesville, Fla., 7-6, 3-6, 7-5, to advance to the semifinals of the Bethesda $25,000 Women's Challenger Series at Holton-Arms School. Carol Christian of Highland, Calif., upset the No. 2 seed, Michelle Jaggard of Sydney, 6-3, 6-4 . . .

Yannick Noah and his wife Cecilia, a Swedish model and singer, have agreed to a separation and will file for divorce, lawyers announced.

Noah, 27, has taken several months off from tennis since losing in the first round of the Bordeaux Open this summer. Noah denied rumors he was thinking of retirement.