Five-time Grand Slam champion Sharapova, who won at Flushing Meadows in 2006, struggled with her serve at times in the opening set but Schnyder failed to take full advantage.
Serena Williams must navigate a heavily American quarter of the draw including sister Venus to complete a calendar sweep of the four Grand Slams, while two sensational quarter-finals loom on the men's side after Thursday's U.S. Open draw.
Maria Sharapova's outfit sparkled even if her play sometimes didn't during her grand slam return at the US Open but the former world number one's run to the fourth round showed she is back in business on and off the court.
The final will pit power against power when Andreescu takes on her childhood hero Williams, who claimed her first US Open title in 1999 before Andreescu was even born.
After a flawless first set from Mertens the tide turned in Andreescu's direction in the second when she fired a cross-court forehand winner to break the Belgian at love and take a 4-2 lead.
Prajnesh Gunneswaran got a taste of high-level tennis in his first-round exit from the US Open, losing to world number five Daniil Medvedev in 84 minutes, in New York, on Monday.
Simona Halep did something no one else was able to do against Serena Williams at the US Open on Wednesday but still could not stop the world number one from advancing to the semi-finals of the year's final Grand Slam.
Peng Shuai of China upset fourth-seeded Agnieszka Radwanska of Poland 6-3, 6-4 in a shocking start to the women's second round of the U.S. Open on Wednesday.
Federer has been brandishing an aggressive style of play, forcing the action and rushing in to half-volley weak second serves to forge an advantage.
Serena Williams has been vulnerable this Grand Slam season but unbeaten in the year's first three major championships.
Rafael Nadal has pulled out of next week's ATP Tour Finals in London due to injury, so Novak Djokovic will end the year as world number one having returned to the top of the rankings on Monday.
Defending champion Marin Cilic had to battle hard before beating Kazakhstan's Mikhail Kukushkin 6-7, 7-6, 6-3, 6-7, 5-1 in their 3rd round match of the US Open on Friday.
Tennis must consider shortening matches in a bid to engage with a younger fanbase and avoid losing television viewers, according to former ATP Player Council president and doubles specialist Eric Butorac.
Roger Federer launched his bid for an 18th grand slam title with a workmanlike 6-3, 6-4, 7-6 (4) win over Marinko Matosevic to ease into the second round of the US Open on Tuesday, with NBA great Michael Jordan leading the cheering.
Sumit Nagal lived a dream when he took on one of the greatest players of all time at a packed Arthur Ashe stadium in a US Open first-rounder.
The German sixth seed, who has not won a title since winning at Flushing Meadows last year, never got into the match against the 2016 WTA newcomer of the year.
Serena Williams has never been one to approach the net until it is time to exchange post-match pleasantries with an opponent but on Thursday she broke from tradition and it paid off with a trip to the U.S. Open final.
Overpowers Svitolina to reach second straight US Open final
'I had to learn to be patient. I am a different player and a different person than I was three years ago. I still believe I can repeat that 2015-2016 streak when I won four slams in a row, I wouldn't be here if I didn't.'
'Almost three hours. It's midnight now. Credit to John for putting up a great battle'
The World number was pitted against Spaniard Albert Costa in Wednesday's draw.
The former US Open champion has withdrawn from this year's tournament because of a lingering wrist injury.
Sania lost to third-seed Sesil Karatantcheva of Bulgaria in the finals of the Bernstein Tennis Tournament.
47-year-old Martina Navratilova and Lisa Raymond were also picked in the U.S. team for their Fed Cup first-round tie against Slovenia.
Flavia Pennetta beat unseeded Italian compatriot and childhood friend Roberta Vinci 7-6(4) 6-2 to win the US Open women's singles title on Saturday.
World number two Roger Federer launches his bid for a sixth US Open title, and his first since 2008, with a first-round match against Argentina's Leonardo Mayer at the Arthur Ashe Stadium on Tuesday.
Switzerland's Stanislas Wawrinka rose to the occasion once again to beat Novak Djokovic with a scintillating display to win his first US Open and third Grand Slam title.
'I have seen myself as nothing but an Indian,' Prakash Amritraj tells Ashish Magotra.
Nadal claimed his second grand slam title of the year -- having won the French Open in June -- beating No. 28 seed Kevin Anderson of South Africa in straight sets 6-3, 6-3, 6-4 at Arthur Ashe Stadium in Flushing, on Sunday.
The 2003 US Open will be played at the USTA National Tennis Center in Flushing, New York, from August 25 to September 7.
Nadal will next face Ukraine's Alexandr Dolgopolov.
While the year's final grand slam is just underway tennis fans have circled Sept. 8 when, if all goes to form, Nadal and Federer would meet to decide a place in the final.
Seven years after their first Grand Slam final meeting, Novak Djokovic and Roger Federer stand on the verge of a third showdown at the highest level.
Former world number one Maria Sharapova lost her second consecutive first round match with a 6-4, 6-4 defeat to Japan's Naomi Osaka at the BNP Paribas Open in California on Wednesday.
Images from the US Open final played at Flushing Meadows on Sunday
IMAGES from Day 9 of the US Open matches played on Tuesday
Top-seeded world number ones Serena Williams and Novak Djokovic help launch the US Open tennis championships on Monday's opening day of the year's final Grand Slam.
Rafael Nadal boasted after his first-round US Open match that he had a video recording of being the first player to hit a ball in practice under the new roof at Arthur Ashe Stadium.