Serena Williams passed her toughest test so far at the US Open to beat Karolina Pliskova and reach the semi-finals for a ninth consecutive time.

Williams, seeded 17th, trailed 4-2 in the first set but hit back to win 6-4 6-3 against the Czech eighth seed.

The American, 36, is seeking her first Grand Slam trophy since giving birth to daughter Olympia last September.

If she wins it would be her 24th major title – equalling Australian Margaret Court’s all-time record.

“The crowd was really rooting for me and I felt so bad because everyone out here was cheering and I wasn’t winning so I thought I had to try harder,” said Williams.

“I really feel like right now I’m playing free. I was having a baby this time last year so I have nothing to prove.”

Williams faces Latvia’s 19th seed Anastasija Sevastova, who knocked out defending champion Sloane Stephens earlier on Tuesday, in the last four on Thursday.

• Relive Williams’ win over Pliskova
• Champion Stephens knocked out of US Open
• Federer needs to play more tournaments – Cash
• Djokovic targets 12th US semi-final – day 10 preview

Crowd helps Serena turn match around

Williams is only playing her eighth tournament since becoming a mum, but had only dropped one set – in her last-16 match against Latvia’s Kaia Kanepi – on her way to the last eight.

However, having lost to Angelique Kerber in the Wimbledon final, there were question marks how she would fare against another high-calibre player.

After a slow start against the big-serving former number one, Williams grew into the match as she responded to some vociferous backing from Arthur Ashe Stadium.

The six-time US champion won eight games in a row – four at the end of the first and four at the start of the second – to help her secure victory in one hour 26 minutes.

It was her first win over a top-10 ranked player since beating Britain’s Johanna Konta at the 2017 Australian Open.

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Pliskova left to rue missed chances

Williams suffered a chastening defeat by Pliskova, who had a brief spell as world number one last year, when they last met each other in the 2016 US Open semi-finals.

And it looked like she could face more trouble when 26-year-old Pliskova threatened to go a double break up in the first set.

The tall Czech was unable to convert either for a 4-2 lead with her service game next – and that was the momentum-changing moment in what had promised to be an engrossing match.

Williams upped her number of winners as Pliskova’s unforced error count grew, a pattern which continued in the early part of the second set.

Little danger seemed apparent for Williams when Pliskova clawed one break back for 4-1, only for the tension to grow when the world number eight moved 40-0 ahead on the American’s serve in the seventh game.

Williams responded with some huge serving to see off four break points, averting danger for a 5-2 lead and allowing her to wrap up victory in her next service game.

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Roger Federer needs to play more tournaments following his shock US Open exit or consider retirement, says former Wimbledon champion Pat Cash.

The 20-time Grand Slam champion, 37, lost to world number 55 John Millman in the fourth round.

Swiss Federer has scaled down his playing commitments, instead building his schedule around specific events.

“Eventually it catches up with you and you get unexplainable losses and bad performances,” said Cash.

He told BBC Radio 5 live: “It will happen once in a while and then a little bit more and a little bit more. Then you say either play a little bit more or retire.

“He may or may not make that decision this time. I’m not saying retirement is imminent but he might think enough is enough.

“When you come towards the end of the career these losses just happen and you scratch your head and ask ‘what went wrong there?'”

• Listen: Federer may feel he’s had enough – Cash
• Del Potro beats Isner to reach semi-finals
• Williams hits back to beat Pliskova
• Champion Stephens knocked out by Sevastova

Second seed Federer was targeting a sixth US Open victory, having not won at Flushing Meadows since 2008.

But, after winning the opening set, he produced an erratic display to lose to Australian Millman in four sets.

Federer had never lost previously at the US Open to a player ranked outside the world’s top 50.

He skipped the clay-court season for the second successive year, returning to play two grass-court tournaments before losing to Kevin Anderson in the Wimbledon quarter-finals.

That was his earliest exit at the All England Club since losing in the second round to Ukraine’s Sergiy Stakhovsky in 2013.

He responded by taking another break before playing one tournament in Cincinnati, where he lost in the final to Novak Djokovic, in the build-up to the US Open.

Federer says he understands being regularly asked about retirement, having faced with this line of questioning for “like nine years”.

Last week he insisted he was not ready to give up.

Doubts have been voiced before about his longevity, notably when he went almost five years between winning Slams after his Wimbledon triumph in 2012.

Yet, after taking the second half of the 2016 season off with a knee injury, he has returned to win three more majors and also became the oldest world number one in February this year.

Will Federer win another Grand Slam title?

BBC Sport tennis correspondent Russell Fuller

Let’s not forget that Federer will remain the world number two at the conclusion of these championships, but the match conjured up memories of his fourth-round defeat by Tommy Robredo in New York in 2013.

Then, Federer had back problems and was five years younger. I thought he was unlikely to win another Grand Slam and I am so delighted he proved me wrong three and a half years later with his astonishing victory at the 2017 Australian Open, which paved the way to two further Slam triumphs.

I do not want to make the same mistake again. When he is refreshed at the start of the year in Melbourne, or on the grass of SW19, who knows?

But bear in mind that Ken Rosewall is the only man in the Open era to have won a Grand Slam after celebrating his 37th birthday.

In this phenomenal era, and over five sets in the sapping humidity of New York, we should not be at all surprised Federer has fallen short, but we are allowed to feel a little glum.

• Djokovic targets 12th US semi-final – day 10 preview
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Former champion Juan Martin Del Potro reached his second successive US Open semi-final with a hard-earned 6-7 (5-7) 6-3 7-6 (7-4) 6-2 win over John Isner.

World number 11 Isner took the opening set on a tie-break before the world number three levelled following a break in the fourth game of the second set.

The Argentine won the next tie-break and then broke in the second game of the fourth set en route to victory.

The 2009 champion will face Rafael Nadal or Dominic Thiem on Friday.

More to follow.

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