US Open 2018: Serena Williams into semis; Sloane Stephens, John Isner make exit

New York: Six-time champion Serena Williams shook off a sluggish start to power past eighth-seeded Karolina Pliskova 6-4, 6-3 on Tuesday and into the US Open semi-finals.

The US superstar, chasing a record-equalling 24th major title, surrendered an early break to Pliskova, but she roared back with a streak of eight straight games to put away the opening set and take a 4-0 lead in the second against the woman who beat her in the semi-finals at Flushing Meadows in 2016.

"I just wanted to play better," Williams said of her mindset as she trailed 3-1 in the first. "I was thinking, you know, I can play better, so that was the good news."

Williams did indeed cut down on the errors, and came up with the big serves when she needed them.

She finished the match with 13 aces and will take on Anastasija Sevastova for a place in the final after the 19th-seeded Latvian toppled defending champion Sloane Stephens 6-2, 6-3.

Pliskova, who managed to convert just two of 12 break point chances in the contest, said Williams was simply too good in the important moments.

"She hit all first serves on the break points, which is always a little bit tougher to return," Pliskova said.

"The second set, she was just mixing the serve. She was going a lot of wide. Not much that I could do on the break points."

Williams did drop her serve once in the second, coughing up one of her five double faults on break point as Pliskova narrowed the gap to 4-1.

But when she fell behind 0-40 in the seventh game she rescued herself with 112 mph (180 Km/h) service winner followed by a another service winner and an ace.

Pliskova got another chance when Williams batted a forehand into the net, but another booming serve, a backhand winner and an unreturnable serve saw her safely through the danger.

Serving for the match at 5-3 Williams didn't allow any room for doubt, holding at love with two aces and an overhead smash followed by one last ace.

"I really feel like right now I'm playing free because I was having a baby this time last year, so I have nothing to prove," said Williams, who is seeking her first major title since her daughter Olympia was born on September 1 of 2017.

With a win she would break out of a tie with Chris Evert for most US Open titles, and equal Margaret Court's all-time record for Grand Slams.

Del Potro ends Isner’s hopes to reach third US Open semi-final

Juan Martin del Potro reached the US Open semi-finals for the third time Tuesday, defeating John Isner 6-7 (5/7), 6-3, 7-6 (7/4), 6-2 and ending American hopes of a first men's champion at the event since 2003.

Argentine third seed Del Potro, the champion in 2009, goes on to face either world number one and defending champion Rafael Nadal or Austria's Dominic Thiem for a place in the final.

Del Potro dropped his first set of the tournament against 11th seed Isner, who was playing in his maiden quarter-final at his home Slam.

Despite that, the 29-year-old was never broken in the 3 hour 31 minute match where Isner unleashed 26 aces but was undone by 52 unforced errors compared to Del Potro's 14.

"To reach the semi-final again here in New York in my favourite tournament is very special to me," said Del Potro after his eighth win over Isner in 12 meetings.

"To play John in these kind of matches, it's like an epic.

"We fought the whole match and I survived with my serve which was key."

Del Potro admitted the 33-degree temperatures proved a real challenge with the 10-minute heat rule allowing the players a welcome respite after the third set.

"I had a shower, lay on the table and I didn't want to come back again. It was too hot to play tennis," he joked.

Isner said he needed to change his shirt 11 times.

Del Potro carved out the only break point of the first set in the 11th game but was unable to convert before the 33-year-old Isner claimed the tiebreak with his ninth ace.

That was the first set dropped by Del Potro at the tournament.

Another break point came and went for the Argentine in the second set before he deservedly broke through for 3-1, backed up by a love service hold.

He comfortably levelled the contest, giving up just three points on his own serve in the set.