Angelique Kerber has won her eighth consecutive match of the new season, coming from 3-1 down in the final set to beat qualifier Camila Giorgi 6-2, 6-3 to reach the Sydney International final.
The 167th-ranked de Minaur, who will play his first ATP Tour final on Saturday, advanced to the Brisbane International semifinals last week.
Naomi Osaka of Japan celebrates after match point against Angelique Kerber of Germany (not pictured) on day two of the U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center.
It is an impressive start to the new year for Kerber, having won her four matches at last week's mixed teams Hopman Cup in Perth before her three victories in Sydney.
"I think I shot myself in the foot with my serving in the first [set] and didn't give myself an opportunity to be dominant on service games", said Barty, who is now one win from becoming the first Australian woman to win the Sydney International since Alicia Molik in 2005.
"I just tried to play my tennis like I've played the last years and I'm so happy to be in the final in the first tournament of the year", Kerber said in a court-side interview. "It's nice to be able to get a lot of matches here and be rewarded with the final". "I've had a great off-season and I'm working hard".
It's just the beginning of the year.
She will face Ashleigh Barty, who beat fellow Australian Daria Gavrilova 3-6, 6-4, 6-2 in the other semifinal.
"For us, it's ideal". Defending champion Mertens suffered a second-set blip against Watson, victor in 2015, but recovered to earn a 6-4 1-6 6-2 success.
Italian fourth seed Fabio Fognini set up a semi-final against qualifier Daniil Medvedev following a 7-6 (7-4), 6-7 (7-4), 6-2 marathon against Frenchman Adrian Mannarino.
Paire sent down 17 aces against the world No.25 to advance to his second semi-final in a week, after reaching the last four in India.