Ash Barty and Angelique Kerber set up a blockbuster Western and Southern Open semi-final – but Petra Kvitova suffered a fitness scare ahead of the US Open.

World number one Barty won 6-2 6-4 against French Open champion Barbora Krejcikova, who she also defeated in the fourth round during her Wimbledon title run last month.

Kerber and Kvitova then battled it out for the right to tackle Barty in the last four, but a quarter-final that was building momentum ended prematurely when two-time Wimbledon winner Kvitova found a recent stomach problem too troublesome.

Kerber was 6-4 3-3 ahead in the contest, having recovered from being an early break down in the second set, getting her game together as 31-year-old Kvitova began to suffer.

During an injury timeout, which Kvitova called after the fifth game of the second set, the Czech received a massage to her abdominal region and played on briefly, before walking up to the net to concede the match.

It remains to be seen whether the setback in Cincinnati might have consequences for her involvement at Flushing Meadows, with the US Open due to begin on August 30.

Kvitova wrote on Twitter minutes after coming off court: "I stayed out there as long as I could. I have been struggling with a stomach issue for several days and unfortunately couldn't finish the match today. Good luck Angie - sorry to end it that way - and can't wait for the next battle with you."

Kerber said in an on-court interview: "Of course, this is not the way you would want to win the match. I hope Petra will recover as fast as possible and be ready for the US Open.

"I know she's a big fighter and fights until the last points, so I hope she is not too bad and hope she is fine really, really soon."

Kerber has now won 14 of her last 15 matches, with the exception being her Wimbledon semi-final defeat to Barty.

Barty got through in a more routine manner on Friday, albeit having to battle from a break down in the second set against the fast-rising Krejcikova, who has shot up from 65th in the world to a career high of number 10 this year.

Barty said: "Barbora served exceptionally well early in that second set and was able to take advantage of a service game of mine where I just hit too many second serves.

"I wanted to try and get a little bit more positive on her service games and then try to get myself in the points a little bit more. I'm happy to run away with that one in the end."

Rafael Nadal has curtailed his 2021 ATP Tour season due to a long-term foot injury but insists he is working towards a return.

Nadal, 35, has played only twice since losing the French Open semi-final to Novak Djokovic in May.

The Spaniard skipped Wimbledon and the Tokyo Olympics and had withdrawn from this week's Western and Southern Open before the US Open.

Nadal will not now compete in the final major of the season either, while great rival Roger Federer is also out for an extended period.

It clears a path for Novak Djokovic – the third man tied on 20 grand slam titles – to win a record-breaking 21st championship at Flushing Meadows while completing a remarkable calendar Grand Slam.

Djokovic could yet have competition again in the years to come, though, as Nadal is not yet calling time on a glittering career.

In making his latest announcement on social media on Friday, Nadal revealed he has been dealing with his injury since 2005 but has not let it halt him yet.

"Hello everyone," he wrote on his Twitter page. "I wanted to inform you that unfortunately I have to end the 2021 season.

"Honestly, I've been suffering a lot more than I should with my foot for a year and I need to take some time.

"After having discussed it with the team and family, this decision has been made and I think it is the way forward to try to recover and recover well.

"It's a year that I've missed things that matter a lot to me, like Wimbledon, like the Olympics, how the US Open is going to be now, like many other events that are also important to me.

"And in view of the fact that during this last year I have not had the ability to train and prepare and compete in the way that I really like to do it, in the end, I come to the conclusion that what I need is time to recover, change a series of things, try to understand what has been the evolution of my foot in recent times.

"It is not a new injury, it is an injury that I have had since 2005 and it has not prevented me from developing my sports career during all these years.

"If it is true that I have had a season where things are not going as they should, as we would all like, it is time to make decisions, seek a slightly different type of treatment to find a solution to this problem or at least improve it in order to continue to have options for the next few years.

"I have the maximum enthusiasm and predisposition to do whatever it takes to recover the best possible form to keep competing for the things that really motivate me and the things that I've done all these years.

"I am convinced that with the recovery of the foot, and obviously a very important daily effort, this can be achieved. I will work as hard as I can to make it happen.

"Thank you in advance for all the support, understanding and all your expressions of affection that are very important and more in difficult times like these.

"I promise you what I am going to do is work hard to try to continue enjoying this sport for a while longer. A big hug to all."

Nick Kyrgios, John Isner and Reilly Opelka have been named by Team World captain John McEnroe as his final three picks for the Laver Cup.

The trio join Denis Shapovalov, Felix Auger-Aliassime and Diego Schwartzman for the team event which runs from September 24-26 at TD Garden in Boston.

Laver Cup newcomer Opelka rose to a career-high world number 23 ranking en route to his first ATP Masters 1000 final in Toronto and defeated world number three Stefanos Tsitsipas, who will play for Bjorn Borg's Team Europe.

Isner, who has featured for Team World since the inaugural event in 2017, reached the semi-finals in Toronto and claimed his 16th ATP Tour title in Atlanta at the start of August.

He described the Laver Cup as "a highlight of my year", adding: "To be on a team with guys we're normally competing against is so different and so much fun. We come together so well as a group, the chemistry is awesome and it's such a great environment to be part of."

Australian firebrand Kyrgios is a striking inclusion in Team World's roster, while Team Europe will be without their big three: Roger Federer, Rafael Nadal and Novak Djokovic.

Federer and Dominic Thiem were expected to take part in this year's event, though both were forced to withdraw with injuries.

However, Borg's men still boast six of the world's top 11. World number two Daniil Medvedev leads the line-up, with Tsitsipas and Tokyo Olympics gold medallist Alexander Zverev for company.

Casper Ruud, who collected a 14th win in his last 15 completed matches on tour when he beat Opelka on Wednesday, will feature, while Andrey Rublev and Wimbledon finalist Matteo Berrettini complete the six-man team.

Team Europe have landed the title in each of the three editions of the tournament so far, with Prague, Chicago and Geneva having served as hosts.

Two-time US Open champion Venus Williams has been given a wildcard entry to participate in this year's tournament.

The 41-year-old American will be making her 23rd appearance at Flushing Meadows and a 15th in a row.

Williams, who has not missed a grand slam since Wimbledon in 2013, is one of eight wild card selections for the women's singles draw.

Former semi-finalist CoCo Vandeweghe and Emma Navarro are joined by teenagers Hailey Baptiste, Ashlyn Krueger, Caty McNally and Katie Volynets.

Storm Sanders has been awarded a reciprocal wild card in agreement with Tennis Australia.

Naomi Osaka will be looking to defend her title in New York, with the main draw scheduled to run from August 30 to September 11.

Defending US Open champion Dominic Thiem will miss this year's tournament with a wrist injury that will rule him out until 2022.

The 27-year-old suffered the setback at the Mallorca Open in June and also withdrew from Wimbledon and the Tokyo Olympics.

World number six Thiem won his maiden grand slam at Flushing Meadows last year with victory over Alexander Zverev in a five-set thriller, but he will not get the chance to defend his title in New York.

"Having spent a week training on court, I still felt pain in my wrist and I knew that it was not 100 per cent," Thiem posted in a statement on his Twitter page on Wednesday.

"I went to see my doctors again and we have decided to follow a conservative treatment, giving the injury more time to recover.

"I'm very disappointed not to be able to defend my US Open title and to miss the rest of the season.

"But I know this is what I have to do. I have a long career ahead of me, so I will only come back once I'm fully recovered and in good shape to compete."

Thiem has a record of nine wins and nine losses this season, with his best result coming at the Madrid Open in May when reaching the semi-finals.

Twenty-time grand slam winner Roger Federer last week pulled out of the US Open, which runs from August 30 until September 12, after undergoing knee surgery.

Simona Halep says she is no longer afraid of coronavirus after getting vaccinated and hopes more players will take the jab.

Halep opted against travelling to New York for the US Open last year due to the pandemic before testing positive for COVID-19 last October.

The two-time grand slam champion missed the French Open and Wimbledon this year with a torn calf muscle, but claimed her first win since May at the Western and Southern on Tuesday, beating Magda Linette 6-4 3-6 6-0.

Former world number one Halep revealed she feels much more comfortable back on the circuit after taking the vaccine.

"I don't feel afraid, because I am vaccinated," The Romanian told reporters. "I feel protected. I am taking care. I wear the mask when many people are around."

Players have faced strict protocols since the tour resumed, with the challenges of staying in biosecure bubbles and playing in empty stadiums last year before restrictions gradually eased.

Stefanos Tsitsipas this week said that he would only take the vaccine if it becomes mandatory to play on the ATP Tour after Novak Djokovic stated he hopes it will not become a requirement to take the jab.

Halep hopes fellow professionals take the vaccine so they do not face such strict protocols.

"I don't like the bubbles," Halep said. "I feel very stressed when I'm in the bubble.

"So if people can vaccinate more and more (it) will be better, because we will not have more restrictions anymore."

Simona Halep, a three-time runner up in Cincinnati, claimed her first win since May by overcoming Magda Linette 6-4 3-6 6-0.

The 12th seed has just returned from a calf muscle tear which saw her miss the French Open and Wimbledon, and was defeated by Danielle Collins in Montreal last week.

However, at 1-4 down in the second set she required medical attention for what she described as a "sharp pain" in her right leg, but after a rain delay, the Romanian came out of the blocks to claim a third-set bagel.

Halep progressed to the Round of 32 where she will play American Jessica Pegula.

Sunday's breakthrough National Bank Open winner Camila Giorgo was beaten in straight sets by Pegula 6-2 6-2.

Former world number one Victoria Azarenka needed only one hour and nine minutes to dispose of Ludmilla Samsonova and seal her spot in the last 32.

The Belarussian triumphed 6-2 6-3 over the Russian, dominating her first serve and breaking Samsonova five times throughout the match.

Tokyo 2020 gold medalist Belinda Bencic eased past Marketa Vondrousova 6-3 7-5 continuing her bright recent run, winning in one hour and 27 minutes.

In the final match of the day's play, Caroline Garcia defeated 2017 U.S. Open champion Sloane Stephens 7-6 (9-7) 4-6 6-4 in two hours and 33 minutes.

Dayana Yastremska, Bernarda Pera, Karolina Muchova, Ons Jabeur, Elena Rybakina, Veronika Kudermetova and Coco Gauff were all winners too.

Fifth seed Matteo Berrettini survived an early scare on his return from a thigh injury to progress in the second round of the Western and Southern Open in Cincinnati over Albert Ramos-Vinolas on Tuesday.

The Italian world number eight, making his ATP Tour return since losing the Wimbledon final to Novak Djokovic in July, won 6-7 (5-7) 6-3 7-5.

The Spaniard claimed the first set in a tie-break but Berrettini hit back, sending down 25 aces for the match, winning 84 per cent on his first serve.

"I'm happy for the win. I think I served really well," Berrettini said. "My strokes from the baseline weren't working the way I wanted them to, but I knew from the beginning it's been a long time since I played a match, especially on hard, so I expected to feel a little bit weird."

The match extended to two hours and 20 minutes, with the Italian utilizing his backhand slice in the final set to get the edge.

Berrettini, who has a 33-7 record for the season, will face 12th seed Felix Auger-Aliassime or Karen Khachanov in the third round.

"I'm really happy that I'm going to have the chance to play another match because that's what I need," Berrettini said. "I need to play matches. I need to get in the best shape possible."

Canadian sixth seed Denis Shapovalov suffered a shock early upset, going down to France's Benoit Paire 6-3 4-6 7-5 in the Round of 64.

Former US Open, Wimbledon and Australian Open semi-finalist Grigor Dimitrov toppled 13th seed Roberto Bautista Agut 6-3 6-4 in one hour and 21 minutes.

Fellow seed Cristian Garin also bowed out, losing 4-6 6-3 6-4 to qualifier Tommy Paul, while ninth seed Hubert Hurkacz defeated Alejandro Davidovich Fokina 6-1 6-7 (3-7) 6-1.

Marin Cilic, John Isner, Sebastian Korda, Gael Monfils, Reilly Opelka, Lorenzo Sonego and Frances Tiafoe were the other Tuesday winners.

Isner sent down his 13,000th career ace in his three-set win over Briton Cam Norrie.

Stefanos Tsitsipas has indicated he will only get the COVID-19 vaccine if it becomes mandatory to participate on the ATP Tour.

World number three Tsitsipas revealed he has not yet been jabbed against coronavirus with the tennis world divided on the issue. 

The ATP Tour have publicly encouraged players to get vaccinated, but Novak Djokovic is among the high-profile players to have stated it should not be compulsory to compete.

When asked ahead of the Western and Southern Open if he would get the vaccine, Tsitsipas told a media conference he still had reservations.

"No one has told me anything. No one has made it a mandatory thing to be vaccinated," the Greek said.

"At some point I may have to, I'm pretty sure about it, but so far it hasn't been mandatory to compete, so I haven't done it, no."

Tsitsipas received a bye through to the second round in Cincinnati, where he will play either Sebastian Korda or Laslo Dere.

The 23-year-old, who has a tour-high 45 wins this season, climbed up to third in the world rankings last week and boldly targeted overhauling Djokovic in top spot.

"The rankings are there for a reason. They signify something important," he added.

"I think that the very next step would be the number one spot, which I hope I can get to one day.”

Petra Kvitova was one of a handful of seeded players to take the court at the Western and Southern Open on Monday and came away victorious against a past champion of the event. 

The 11th-seeded Kvitova defeated 2019 Western and Southern winner Madison Keys 7-5 6-4 on a rain-plagued opening day of main-draw play at the tournament outside Cincinnati, Ohio. 

The Czech saved eight of nine break points against her serve while breaking Keys three times, and that was enough to pull out the victory. 

Kvitova's countrywoman Barbora Krejcikova, the ninth seed, made her singles debut at the WTA 1000 event with a 6-3 6-2 rout of Daria Kasatkina. 

Fifteenth seed Elise Mertens also was a straight-sets winner, taking down Nadia Podoroska 6-3 6-4, while 13th seed Jennifer Brady defeated Ekaterina Alexandrova 6-3 7-6 (7-3) in a match that finished shortly before 1 a.m. local time.

Though only Krejcikova was in action Monday, the field in Cincinnati includes nine of the top 10 players in the rankings, led by Ashleigh Barty and Naomi Osaka in their first appearances since the Tokyo Olympics. 

In other matches on the first day of play, Heather Watson defeated Aliaksandra Sasnovich 6-2 3-6 6-1, while Paula Badosa outlasted Petra Martic 4-6 6-4 7-6 (11-9) in a two-hour, 34-minute marathon. 

Yulia Putintseva breezed past Zhang Shuai 6-2 6-0, Jelena Ostapenko beat Tamara Zidansek 7-5 6-1 and Jil Teichmann downed Sorana Cirstea 6-2 6-0. 

Angelique Kerber beat Maria Sakkari 6-2 6-2, with Alison Riske matching that scoreline in a defeat of Leylah Fernandez.

Shelby Rogers won 6-4 2-1 when her countrywoman Danielle Collins retired with an injury. 

Andy Murray looked sharp in his first singles match since Wimbledon, rolling to a straight-sets win over Richard Gasquet at the Western and Southern Open. 

Murray defeated the veteran Frenchman 6-4 6-4 on a rainy opening day in Cincinnati, capitalising on a strong service game to advance. 

Murray had 14 aces to just two double faults and won 81 per cent of points on his first serve while saving four of the five break points he faced. 

A two-time champion at the ATP 1000 event, he will face the winner of Tuesday's match between Alejandro Davidovich Fokina and ninth seed Hubert Hurkacz in the second round. 

Two players who shared a birthday Monday also prevailed on their big day. 

On the day he turned 20, 11th seed Jannik Sinner defeated Federico Delbonis 6-2 7-5, while 10th seed Diego Schwartzman had to work a bit harder on his 28th birthday to down Daneil Evans 6-2 4-6 6-3. 

Elsewhere, 12th seed Felix Auger-Aliassime beat Marton Fucsovics 7-6 (7-0) 6-3, while 14th seed Alex de Minaur rallied to down Filip Krajinovic 0-6 6-4 6-4. 

Fifteenth seed David Goffin fell 6-3 6-3 to Guido Pella in the only seeded upset of the day. 

Other winners Monday included Karen Khachanov, Fabio Fognini, Lloyd Harris, Dominik Koepfer, Benoit Paire, Albert Ramos Vinolas and Mackenzie McDonald. 

Among those set to play their opening matches Tuesday are third seed Alexander Zverev, who will face Harris, and sixth seed Denis Shapovalov, who plays Paire. 

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