Top seed Boris Becker of West Germany thrashed France's Henri Leconte, 6-1, 6-2, to advance to the semifinals of the $500,000 Stella Artois indoor tournament in Milan, Italy.

Earlier, Jimmy Connors rallied to beat West German Carl-Uwe Steeb, 7-5, 6-3, to reach the semis without dropping a set. Unseeded Milan Srejber of Czechoslovakia upset countryman Miloslav Mecir, seeded third, by 3-6, 7-6 (7-4). Yannick Noah of France defeated Australian Pat Cash, the No. 4 seed, 6-4, 6-1.

Today, Becker plays Noah and Connors meets Srejber . . .

In Memphis, unseeded Robert Seguso, extending a string of upsets, reached the semifinals of the U.S. Indoor Championships with a 7-6 (8-6), 7-6 (7-1) victory over No. 5 seed Amos Mansdorf.

Seguso, better known for his doubles play with Ken Flach, lost just 21 points in 12 service games. He had nine aces.

Also advancing to the semifinals were sixth seed Andre Agassi, a 6-4, 6-2 winner over No. 3 seed David Pate, and Kevin Curren, who defeated No. 2 seed Andres Gomez, 3-6, 6-3, 6-4 . .

Unseeded Larisa Savchenko, the Soviet Union's No. 2 player, upset third seed Hana Mandlikova, 6-3, 2-6, 7-6 (7-3) to gain a semifinal berth in the $250,000 Virginia Slims of California in Oakland.

Top-seeded Martina Navratilova and defending champion Zina Garrison also advanced. Navratilova beat Natalia Zvereva, the No. 1 Soviet player, 6-2, 6-0, and Garrison, seeded fourth, defeated Hu Na, 6-2, 6-0.