NEW YORK -Defending champion Sloane Stephens just keeps sailing along. There were only a few more games that Sevastova had to do in sealing the round and Stephens' unforced errors were part of the problem which kept appearing like an unwanted guest at a celebration.
"Defending a title isn't defending". The two will square off in the semi-finals on Thursday, with the 23-time major champion Williams starting as the firm favourite. When you're playing a whole new draw, different seedings, different ranking, there's so much that goes into it. When you go to a tournament, it starts all over again.
Stephens struggled throughout in a match that saw just three of the 17 games played won from the end of the court in the full glare of the sunshine, losing 6-2, 6-3.
Next match, can employ that strategy against No. 19 Anastasia Sevastova of Latvia.
Third seed Stephens was wilting in the heat on what is expected to be the hottest day of a scorching U.S. Open but, after falling 2-0 down at the beginning of the second set, she finally showed some fight. Mertens lobbed Stephens deep into deuce court, but Stephens, in full sprint, ran it down and hit a magnificent cross-court forehand victor with the stunned Mertens racing to the net.
Williams fell behind again in the seventh game, but erased a 0-40 deficit with a 112 miles per hour (180 Km/h) service victor followed by a another service victor and an ace. She made three errors with and Stephens belted a forehand victor for the break and a 4-3 lead.
Elsewhere, five-time US Open champion Roger Federer slumped to a shock defeat at the hands of John Millman in the last 16. The momentum didn't quite carry over. "I can win it maybe next week again". Today, I served well at certain moments, so I think I'm on the right track.
Next up for Williams is a quarter-final meeting with eighth seeded Czech Karolina Pliskova.
"Not because I didn't know my game was there, it was because of the moment", she said. "Since then, we didn't play". I'm not going to dwell on it.
A 10-minute break will be allowed between the second and third sets of women's singles matches if either player requests one.
It hasn't been an outstanding season for Pliskova, who has one tournament victory, but a positive outlook carries her forward.
The 25-year-old said: "The fact that I made it to the quarter-finals and played some really good matches and I just competed as hard as I could, there's a lot to be proud of".