Serena Williams will face fellow former world No.1 Victoria Azarenka for a spot in the US Open final after the mums won through to the last four in contrasting fashion on Wednesday.

Williams come out on top against another tennis mum, rallying back from a set and a break down to defeat Tsvetana Pironkova 4-6 6-3 6-2.

A relaxed and confident Azarenka crushed 16th-seeded Elise Mertens 6-1 6-0 to return to the semi-finals at Flushing Meadows for the first time since 2013.

With no fans in Arthur Ashe Stadium, Belarusian Azarenka amused herself as she smiled and bobbed her head along to music being played during changeovers.

The two-times Australian Open champion fired 21 winners to dominate the Belgian Mertens in their first career meeting in a singles match.

The 31-year-old Azarenka didn’t let her foot off the gas in the second, running Mertens around the court and sealing the win when Mertens dumped a backhand into the net on match point.

It was a much tougher challenge for Williams against Pironkova, who on one point hit a return of serve left-handed and served 20 aces for the match – her most in a match in eight years.

It’s not how you start, it’s how you finish, right?” Williams said.

The unseeded Pironkova, doesn’t even appear in the WTA rankings at all and played with a protected ranking – this was her first tournament of any sort in more than three years, because she left the tour to become a mother.

“It’s unbelievable,” Williams said about Pironkova’s impressive return to competition.

“Wow. I couldn’t even do that.”

Williams, who turns 39 in less than three weeks, has won six US Open titles and is just two wins away from a record-tying 24th grand slam singles title.

Williams defeated Azarenka in the Belarusian’s US Open finals appearances in 2012 and 2013 and holds a 18-4 head-to-head record.

Naomi Osaka, the 2018 champion, will take on Jennifer Brady in the other US Open semi-final with both matches scheduled for Thursday at Arthur Ashe Stadium.