Dominic Thiem will advance into the semifinals of the masters 100 tournament in Indian Wells 2019.
France's Monfils, ranked 19th in the world, pulled out shortly before his scheduled quarter-final against Thiem with left Achilles tendon trouble.
Kecmanovic, who gained entry to the draw after 5th-seeded Kevin Anderson withdrew was the first lucky loser to reach the quarter-finals at Indian Wells since the tournament served as the inaugural ATP Masters 1000 event in 1990.
This Indian Wells is "not over" for Monfils since he will definitely be there on Friday to support his girlfriend Elina Svitolina that will play the semifinal against Bianca Andreescu.
Bencic, who won her third career title at Dubai last month and her first since 2015, next plays the victor of the match between Angelique Kerber and Venus Williams.
Bencic improved to 18-3 this year.
Three seeded players withdrew before the tournament began, clearing the way for Kecmanovic to become a lucky loser and receive a first-round bye.
Angelique Kerber held off a spirited challenge by the unseeded Venus Williams to set up an Indian Wells Open semi-final against Belinda Bencic. "I'm not going to the court with any expectation".
In the women's singles, Belinda Bencic beat fifth seed Karolina Pliskova 6-3, 4-6, 6-3 to reach the semis, ensuring her return to the top 20 in the rankings.
Williams fought off two break points to tie the second set 2-all before Kerber again won three games in arrow for a 5-2 lead.
Williams never got her big serve going, producing just one ace. "It's always tough to play against her".
Reigning Wimbledon champion Kerber meanwhile reached the Indian Wells semi-finals for the third time with her win over Williams in a battle of former world number ones.
With their first ever meeting going the distance, Bencic took the decider to extend her winning run to 12 matches and back up her upset of top seed and defending champion Naomi Osaka in the fourth round.
"We are the one who is closest in our lives, so you can be really straightforward", Latisha Chan said.
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