Roger Federer turned 40 on Sunday amid uncertainty over whether he will grace the stage of a grand slam again.

Both he and Serena Williams, who reaches the same birthday landmark in September, have kept their future plans under wraps.

However, it would come as no surprise now if one, or both, were to retire by the end of the year.

Injuries are taking their toll, and even the greatest champions cannot go on forever.

Stats Perform looked at both Federer and Williams, considering what they may still want to achieve, and their prospects of attaining those remaining goals.

Federer's final fling?

Ahead of his 30th birthday, Federer was asked what it felt like to hit such a milestone.

"Birthdays happen. They're part of life," Federer said. "I'm happy I'm getting older. I'd rather be 30 than 20, to be honest. To me it's a nice time."

A decade on, Federer has good reason to be satisfied with life as he chalks up another decade. Family life is good, he'll never need to borrow a dollar, and he has advanced from 16 grand slams to 20.

But the knees would sooner be 30 than 40, and Federer, remarkable sportsman though he is, looks to have entered the lap of honour stage of his career – if he can even complete such a lap.

Two knee operations in 2020 were followed by a setback that ruled him out of the Olympics and will also keep him sidelined for the Toronto and Cincinnati tournaments before the US Open.

Will Federer make it to Flushing Meadows, where he won five successive titles at the height of his career? There has to be doubt over that, and should he indeed be an absentee in New York, what is there left to target? The Laver Cup, perhaps, a tournament in which he is financially invested and which is due to be played in Boston in late September.

Would he play on in 2022? Could he tolerate more long road trips without his family, living in a tennis bubble?

Target: Federer has never settled for second best, and it may have dawned on him at Wimbledon that in all probability he no longer can win a grand slam. Losing a 6-0 set to Hubert Hurkacz on the way to a quarter-final exit would have hurt. The hunger does not go away after 20 grand slams, but Federer's battle-weary body is sending him messages. He will want to go out on his own terms, which means getting fully fit.

Prospects: Assuming the knee issue is not a major problem, and more of a niggle, then Federer could still play the US Open, Laver Cup, Indian Wells and Paris Masters this year. If the mind is willing but the body does not comply, however, then it would not be a shock to see him call time before the Australian Open comes around in January.

Serena still one short of Court

From precocious teenager to queen of the tour, Williams' tennis journey has been a 25-year odyssey and there is nobody more driven to succeed than the great American.

It is an intense frustration that she remains rooted on 23 grand slams, one short of Margaret Court's record haul, and the four grand slam final losses she has suffered while on that mark have been cruel blows.

As her 40th birthday approaches on September 26, prospects of matching Court are fading. The leg injury that cruelly forced her out of Wimbledon in the first round was a harrowing turn of events, given she looked primed to be a big title challenger in London.

She is becoming less of a factor when looking at title favourites, but Williams is still capable of beating top players, still a threat wherever she shows up. It comes down to whether the body lets her chase her goals, and whether the pain of so many near-misses in recent years persuades this great champion the exertion is no longer worth prolonging.

Target: The 24th slam has been the must-have for Williams. Tour titles feel like an irrelevance, and Williams has won just one of those since January 2017, her calendar built around peaking for the majors since returning from giving birth to daughter Olympia.

Prospects: Beating Aryna Sabalenka and Simona Halep at the Australian Open demonstrated Williams still has the game for the big stage, and a semi-final defeat to Naomi Osaka, to whom she has now lost in three of four encounters, should not particularly detract from that. Wimbledon felt like a golden opportunity, with a host of major rivals absent and others struggling for form. There is no doubt she felt that way. Getting to 24 – and beyond – has shifted from feeling like an inevitability to being an odds-against chance now.

Novak Djokovic led the ATP Tour's birthday message to Roger Federer, as the Swiss star celebrated his 40th on Sunday.

Federer, ranked ninth in the world, made his professional debut in 1998. He has gone on to win 20 grand slam titles, a feat matched by only Rafael Nadal and Djokovic, who tied his rivals with his triumph at Wimbledon in July.

His first grand slam title arrived at Wimbledon in 2003, when Federer was aged just 21. In 2004, 2006 and 2007, he won three out of the four majors on offer.

Federer has six Australian Open titles, eight Wimbledon triumphs, five US Open wins and one French Open success to his name, while he has also won six ATP Tour Finals.

Djokovic and Federer have met 50 times on the ATP circuit, with the Serbian shading their head-to-head record with 27 wins – their most recent encounter coming in the semi-finals of the 2020 Australian Open.

Yet it was he who led the tributes to a true great.

"Hey Roger, happy 40th birthday – 40th, wow, what a milestone! You still keep on inspiring and thriving on the court, off the court, inspiring all of us," Djokovic said in a video posted to the ATP Tour's official Twitter account.

"It's been a huge honour to share the tennis court and the tennis circuit with you in the last 15 years.

"Hopefully you can still keep on playing. The sport needs you, of course, and thanks for everything you have done and thanks for showing us that even at that age we can play at a very high level.

"All the best, I wish you health and love and happiness with your close ones and thanks again for everything."

Kei Nishikori, who has beaten Federer three times and suffered eight defeats to the Swiss, was next up.

"Roger, happy 40th birthday!" Nishikori said.

"You are still my idol, I always looked up to you and I hope we can play a couple more times and I hope you can still keep going and win grand slams."

World number 15 Felix Auger Aliassime shares his birthday with Federer, though the Canadian is some 19 years junior.

"Hey Roger, the big 40, while I'm just turning 21," he said. "It's amazing for the sport that I'm playing and you're still playing at the same time.

"I hope I'll still be playing when I'm 40 as well. Thank you for everything you've been doing for tennis, it's so good to still have you around, I hope you have a great birthday."

Top seed Elise Mertens was knocked out as Daria Kasatkina won through to the Silicon Valley Classic final where she will meet Danielle Collins.

Fourth seed Kasatkina swept aside world number 17 Mertens in one hour and 21 minutes, winning 6-3 6-2 in San Jose.

The 24-year-old Russian progresses to her fourth final of the WTA season, claiming her third win from four meetings against the Belgian.

The win was former top 10 player Kasatkina's fourth triumph over a top 20 player this year.

Kasatkina broke Mertens three times throughout the match, never dropping her own serve, sending down 4-2 aces.

Mertens was not helped by seven double faults throughout the match, with Kasatkina saving the Belgian's only break point.

Kasatkina will face American seventh seed Danielle Collins who was too good for Ana Konjuh. Collins only needed 52 minutes to win 6-0 6-2 over the Croatian qualifier.

Konjuh struggled on serve throughout, with Collins breaking her six times, while she only won one from 17 points on her second serve.

American Mackenzie McDonald toppled former champion Kei Nishikori in three sets to claim a spot in the Citi Open final on Sunday against fifth seed Jannik Sinner.

McDonald triumphed over unseeded Japanese Nishikori 6-4 3-6 7-5 in two hours and 45 minutes in Washington DC.

The 26-year-old American's win qualifies him for his maiden ATP Tour final, sending down 4-1 aces while getting a slight edge with his return.

Nishikori, who triumphed in DC in 2015, fought back when trailing a set and a break in the second with four straight games to send it to a third.

The 31-year-old Japanese appeared to have the momentum but McDonald responded to outlast him.

McDonald will face 19-year-old Sinner who overcame another emerging talent, American 20-year-old Jenson Brooksby, 7-6 (7-2) 6-1.

Sinner becomes the first Italian to reach the final in DC and also progresses to his first ATP 500 decider.

The Italian continued his fine run in DC, having not yet dropped a set all tournament, marking a return to form after being on a four-match losing run.

Sinner saved three set points at 5-6 in the first to fight back to take the lead in a tiebreaker before a dominant second.

It was by no means certain the rescheduled Tokyo 2020 Olympics would even go ahead, such was the uncertainty caused by the coronavirus pandemic.

But go ahead they did and now here we stand on the eve of the closing ceremony in the Japanese capital.

They have certainly been a Games like no other and we all hope future Olympics will not be held under such unusual circumstances, and judging the success of Tokyo 2020 is no easy feat given the measures to do so are too arbitrary.

Having said that, here are the highs of the Games and some of the lows, too.

The highs…

WARHOLM AND MCLAUGHLIN HAMMER THE HURDLES

Karsten Warholm revelled in bringing the "wow" factor to the men's 400m hurdles, and rightly so. The Norwegian became the first man to break the 46-second barrier – running an astonishing 45.94 seconds to smash his own world record, five weeks after breaking a benchmark held by Kevin Young for 29 years. A day later, Sydney McLaughlin battered her own world record in the women's race, clocking in at 51.46s.

VAN VLEUTEN'S HEARTWARMING TRIUMPH

Five years ago in Rio, Annemiek van Vleuten was on course for victory in the women's cycling road race until a high-speed crash left her with minor fractures to her spine. To make matters worse, the Dutchwoman made headlines for celebrating what she thought was victory in the same event here in Tokyo – only to realise she had finished second behind runaway winner Anna Kiesenhofer. But finally, her golden moment arrived in the women's time trial – at the age of 38 years and 293 days, she became the third-oldest woman to win Olympic gold for the Netherlands.

SWIMMING STARS PROVE THERE'S LIFE AFTER PHELPS

Michael Phelps is an Olympics legend and no one can lay claim to more than the 23 golds or 28 overall medals he accrued over between 2004 and 2016. But a stellar cast this year proved swimming is in a very strong position. Emma McKeon took home seven medals (including four golds) – the joint-most of any woman at a single Games – while Ariarne Titmus' 200m and 400m free double was memorable, particularly her win over the great Katie Ledecky in the latter race. Caeleb Dressel took five golds to show his potential as Phelps' heir apparent, while Adam Peaty stunned again for Great Britain. It was some week in the pool.

THOMPSON-HERAH DOES THE DOUBLE-DOUBLE

Elaine Thompson-Herah announced herself to the world stage with a 100 and 200m sprint double at Rio 2016 but injuries in the intervening years stemmed her momentum a little. However, she peaked at the perfect time in Tokyo and backed up her double from Brazil – becoming the first woman to repeat on the 100 and 200m. Indeed, only Usain Bolt had ever previously done so.

THE AZZURRI'S GOLDEN HOUR

There was a shock in the men's 100m final where the unheralded Marcell Jacobs started the post-Bolt era with gold. That followed on from countryman Gianmarco Tamberi having minutes earlier shared high jump glory with Mutaz Essa Barshim. There were hugs aplenty as Italy, surely celebrating their greatest night at an Olympics, won two athletics golds at the same Games since Athens in 2004.

NEW EVENTS CATCH THE IMAGINATION

One of the most fascinating aspects of any Olympics is the new sports and categories that get added to the programme. At Tokyo 2020, skateboarding, surfing and climbing have all attracted new and younger audiences to the Games – while the addition of mixed triathlon and the mixed 4x400m track relay have been successes.

BILES' INSPIRATIONAL MESSAGE

On the one hand, the fact we saw so little of Simone Biles and some of the reprehensible bilge aimed her way over the decision to pull out of the women's team event after just one rotation and then miss four individual events can be seen as a negative. But, on the other hand, the fact that she came back to take bronze on the balance beam and use her platform to promote the importance of protecting mental health has to be seen as a high. It takes bravery and courage in her position to speak on such matters. Kudos to you, Simone.

And the lows…

EMPTY STADIUMS AN ENDURING IMAGE

Let's start with the obvious here and something that has been spoken about pretty relentlessly. The absence of fans has had a huge cost on the atmosphere at these Games. Magical moments and career peaks played out in front of huge, empty stadia has undoubtedly been a huge negative. Many will take the fact we got here and managed to hold a Games at all as a positive. And it is. But at times, the whole thing felt a bit… meh.

TENNIS' HEADLINE ACTS FAIL TO DELIVER

With so many of the top male players opting to skip Tokyo, there was a big focus on Novak Djokovic and the next checkmark on his quest for a rare Golden Slam (only Steffi Graf has ever done it). The Serbian fell short, dropping out at the semi-final stage then getting a little stroppy. Big things were also expected of Naomi Osaka – a home hope and the 'face of the Games'. She made it as far as round three before going down to Marketa Vondrousova.

THE TSIMANOUSKAYA SAGA

One of the ugliest stories to emerge from the Games was the story of Belarusian runner Krystsina Tsimanouskaya, who refused to board a flight after allegedly being taken to the airport against her will following her public criticism of her team's organisation on social media. Tsimanouskaya competed in only one event and claimed she was entered into a 4x400m relay despite never racing in the discipline, suggesting that was a result of members of the team being considered ineligible due to not completing enough doping tests. The Belarusian Sport Solidarity Foundation indicated Tsimanouskaya feared for her life upon returning to Minsk. The country is under the authoritarian leadership of president Alexander Lukashenko, whose son Viktor heads the national Olympic committee (NOC). Both men were banned last December from attending Tokyo 2020. The whole thing has been really rather unsavoury.

Top-seeded Elise Mertens rolled into the semi-finals at the Silicon Valley Classic with a 6-3 7-6 (10-8) win over Yulia Putintseva on Friday. 

The Belgian already has collected a pair of grand slam doubles titles this year at the Australian Open and Wimbledon and is seeking her seventh career singles crown. 

She will need to get past fourth seed Daria Kasatkina to have a chance. 

The Russian outlasted Magda Linette 6-4 3-6 6-4 in her quarter-final and is seeking her third title of 2021 after a three-year drought between wins. 

The other semi-final will match seventh seed Danielle Collins against qualifier Ana Konjuh.

Collins pulled out a tight 7-6 (7-5) 7-6 (7-4) upset of second seed Elena Rybakina to reach the semis in San Jose for the second time. 

The American won her eighth consecutive match after taking the title at Palermo two weeks ago. 

She will meet the only unseeded player remaining, Konjuh, who won the final 10 games of the match to close out Zhang Shuai 3-6 6-2 6-0. 

Kei Nishikori is into his first ATP Tour semi-final in more than two years after defeating Lloyd Harris in straight sets at the Citi Open on Friday. 

By far the most experienced player remaining in the field after Harris' upset of Rafael Nadal on Thursday, Nishikori prevailed 6-3 7-5 to avenge a loss earlier this year in their only prior meeting. 

Nishikori had lost his last six ATP quarter-final matches dating to a trip to the semis in Barcelona in April 2019. 

The 31-year-old next faces Mackenzie McDonald, who downed countryman Denis Kudla 6-3 6-2 to reach his second career ATP semi-final. He has never played in a final. 

In the other semi-final, fifth seed Jannik Sinner will meet wild card Jenson Brooksby. 

Sinner had little trouble with American veteran Steve Johnson in a 6-4 6-2 win as the Italian won 93 per cent of his first-serve points and saved all three break points he faced. 

Brooksby maintained the strong return game that as carried him all week in a 6-1 6-2 upset of 11th seed John Millman. 

The Australian did not manage an ace against the 20-year-old American, who won 54 per cent of the points on Millman's serve and converted five of seven break points. 

At 130 in the world, Brooksby is the lowest-seeded Citi Open semi-finalist since John Isner reached the 2007 final while ranked 416th.

Top-seeded Elise Mertens pulled out a three-set win to reach the quarter-finals at the Silicon Valley Classic as all four matches went the distance on Thursday.

The Belgian, ranked 17th in the world, prevailed 6-2 4-6 6-4 over Kristina Mladenovic of France in two hours and 40 minutes to advance in San Jose.

She moves on to face the eighth seed, Yulia Putintseva, who rallied past Ajla Tomljanovic 3-6 7-5 6-3 as the Australian blew numerous chances to pull ahead by converting only two of 16 break-point opportunities.

In the bottom half of the draw, second seed Elena Rybakina remained on track with a 2-6 6-0 6-2 victory over American Claire Liu in a rematch of a second-round meeting at Wimbledon.

Rybakina next meets seventh seed Danielle Collins, a 3-6 6-4 6-3 winner over US countrywoman Sloane Stephens.

Stephens, the 2017 US Open champion, has slipped to number 67 in the world and was playing in her first tournament since Wimbledon as she prepares for a return to Flushing Meadows.

This time, Rafael Nadal could not summon the third-set magic he needed.

Coming off an epic three-hour duel Wednesday in his first match since the French Open final, Nadal fell to Lloyd Harris 6-4 1-6 6-4 on Thursday at the Citi Open. 

After a short-lived maiden appearance at the Washington, D.C. tournament, the Spaniard will look ahead to Toronto and Cincinnati as he continues his preparations for the U.S. Open. 

Harris, meanwhile, heads to a quarter-final matchup against veteran Kei Nishikori after recording the biggest win of his career. 

Facing Nadal for the first time, the 24-year-old South African slammed 16 aces Thursday and saved four of six break points. 

The 20-time grand slam winner, who battled a foot injury to outlast Jack Sock in his first match, did not have enough to answer the world number 50 on Thursday.

In addition to the top-seeded Nadal's defeat, second seed Felix Auger-Aliassime also went down Thursday, falling 6-3 6-4 to American wild card Jenson Brooksby. 

With their exits, the tournament is now without its top four seeds. 

That leaves fifth seed Jannik Sinner as the top player remaining after he defeated Sebastian Korda 7-6 (7-3) 7-6 (7-3). 

Nishikori advanced by downing seventh seed Cameron Norrie 3-6 6-3 6-3, while John Millman knocked out eighth seed Reilly Opelka 6-3 7-6 (7-4). 

Among unseeded players, Denis Kudla defeated the red-hot Brandon Nakashima 3-6 7-6 (7-3) 6-4, while Steve Johnston eased past Ricardas Berankis 6-2 6-1 and Mackenzie McDonald beat Illya Ivashka 6-4 6-4. 

In addition to Harris-Nishikori, Kudla will face McDonald in the other quarter-final in the top half of the draw, while Sinner meets Johnson and Millman takes on Brooksby in the bottom half. 

Roger Federer has withdrawn from both the Canadian Open and Cincinnati Masters as he continues to recover from a knee injury.

After undergoing two operations on his right knee last year, Federer has competed in just five events so far in 2021.

Most recently, the 20-time Grand Slam champion reached the Wimbledon quarter-finals, before losing in straight sets to an inspired Hubert Hurkacz.

The 39-year-old then missed the Olympic Games due to what he described as a "setback" with his knee.

He was scheduled to appear in Toronto and Cincinnati – where he is a seven-time winner – and step up his preparation ahead of the US Open.

However, his participation in the final Slam of the year is now uncertain after he pulled out of this month's ATP Masters 1000 tournaments.

Another player who will not be competing in Toronto is Alexander Zverev; the Olympic gold medallist also opting to withdraw.

In a statement, he said: "Due to the intense past couple of weeks and my incredible experience at the Olympics, I need to recover so that I can hopefully be at my best for the remainder of the U.S. summer swing.

"It was a difficult decision for me as I have had great memories from Canada and I can't wait to be back next year!"

World number 26 Madison Keys was one of three seeded players to be eliminated in the second round of the Silicon Valley Classic in San Jose on Wednesday.

The American third seed was beaten by China's Zhang Shuai 7-5 7-6 (7-5) in one hour and 39 minutes.

Keys sent down 7-1 aces but Zhang, ranked 51st in the world, was excellent on return, converting four of her 11 break points.

Fourth seed Daria Kasatkina was also in danger after losing the first set but rallied to win 3-6 7-5 6-3 over France's Caroline Garcia.

"It was a tough match. From my point of view, I think the level of the game was very good, and I'm really happy that I was able to turn the match in the second set," Kasatkina said in the on-court interview.

Ninth seed Alison Riske was emphatically beaten by Croatian Ana Konjuh 6-1 6-4 in one hour and 13 minutes, while sixth seed Petra Martic bowed out to Poland's Magda Linette 7-5 7-6 (7-5).

World number three Rafael Nadal outlasted Jack Sock in a three-hour epic to progress through to the third round at the Citi Open in Washington D.C. on Wednesday.

The 20-time Grand Slam winner needed a third-set tiebreaker to beat American wildcard Sock 6-2 4-6 7-6 (7-1).

Sock, who is currently ranked 192nd in the world, stormed back into the match with an early break in the second set after the Spaniard cruised in the first.

In a three-hour-and-a-four-minute match which saw only five breaks, Nadal got the edge winning more on his second serve, while he produced some vintage winners too.

Nadal goes through to the third round where he will face 14th seed Lloyd Harris, who progresses after a walkover with Tennys Sandgren retiring mid-match.

Third seed and world number 18 Alex De Minaur was bundled out by world number 81 Steve Johnson 6-7 (5-7) 6-4 6-2.

Fourth seed Grigor Dimitrov also was dumped out with a 6-2 7-6 (7-4) defeat to Belarussian world number 65 Ilya Ivashka.

Sixth seed Dan Evans went out in straight sets to 20-year-old American Brandon Nakashima 7-6 (7-1) 6-0 while 10th seed Taylor Fritz also bowed out to Denis Kudla 6-4 6-2.

Former winner Kei Nishikori knocked off ninth seed Alexander Bublik 6-2 7-5, while 13th seed Benoit Paire lost in three sets to Mackenzie McDonald.

Fifth seed Jannik Sinner got through in straight sets against Emil Ruusuvuori, along with eighth seed Reilly Opelka and 11th seed John Millman.

Danielle Collins eased past Shelby Rogers 6-4 6-3 to book her spot in the second round at the Silicon Valley Classic on Tuesday.

World number 36 Collins proved too strong for American compatriot Rogers, winning in one hour and 25 minutes.

Collins will meet another American in the second round, with Sloane Stephens up next.

Former U.S. Open winner Stephens beat Caty McNally 6-4 6-2 earlier in the day.

Yulia Putintseva, meanwhile, eased her way into the second round with a 7-5 6-3 win over Croatian Donna Vekic.

Putintseva will face Australian Ajla Tomljanovic in the second round after she fought back to overcome Amanda Anisimova 1-6 7-5 7-5.

France's Kristina Mladenovic won 6-4 6-4 over Han Na-Lae, while American Claire Liu won in three sets against Dayana Yastremska.

Mladenovic will face top seed Elise Mertens, while Liu meets second seed Elena Rybakina.

Second seed Felix Auger-Aliassime survived after an early scare against Andreas Seppi to progress to the third round at the Citi Open in Washington D.C. on Tuesday.

The Canadian world number 15 won 2-6 6-2 6-2 in just under two hours over the 37-year-old Italian.

Seppi had beaten Auger-Aliassime in the opening round at the French Open in June.

Auger-Aliassime this time hit 25 winners, including nine aces, overcoming a slow start where he was broken twice inside the opening six games to win in three sets.

Youngster Jenson Brooksby continued his bright start in Washington D.C. with a 7-6 (7-4) 7-5 win over fellow American Francis Tiafoe.

Australian Nick Kyrgios was bundled out by Mackenzie McDonald 6-4 6-4, while 12th seed Sebastian Korda eased past Vasek Pospisil 7-5 6-4 in the second round.

Seventh seed Cameron Norrie also triumphed in straight sets over Marcos Giron 6-4 6-3, while emerging star Brandon Nakashima celebrated his 20th birthday with a 6-3 6-3 victory over Alexei Popyrin.

Denis Kudla beat Feliciano Lopez to set up a second round date with 10th seed Taylor Fritz, while Tennys Sandgren will meet 14th seed Lloyd Harris after beating James Duckworth in three sets.

Finland's Emil Ruusuvuori beat India's Prajnesh Gunneswaran 2-6 6-1 6-1 and will take on fifth seed Jannik Sinner in the second round.

Ninth seed Alison Riske defeated former top 10 player CoCo Vanderweghe in the opening round of the Silicon Valley Classic in straight sets on Monday.

Riske, who made the Wimbledon quarter-finals in 2019 and is currently ranked 37th in the world, won 6-3 6-4 in one hour and 28 minutes.

In a match which saw eight breaks converted, Riske's ability to win on her first serve proved the difference.

Vanderweghe, who made the Silicon Valley Classic finals in 2012 and 2017, was broken five times and won 58.1 per cent of first serve points.

Croatian sixth seed Petra Martic eased into the second round with a 6-3 6-3 victory over Anastasija Sevastova.

Martic will face Magda Linette next, after the Pole accounted for Dutch qualifier Lesley Kerkhove 6-3 6-3.

Ana Konjuh defeated Marie Bouzkova in a three-set epic which last two hours and 41 minutes, winning 7-5 3-6 7-5.

China's Zhang Shuai won over Emma Raducanu in straight sets, along with Carolina Garcia who defeated Emina Bektas.