WIMBLEDON, England - Every champion must be tested. This is what we know. And as Serena Williams continues her quest for No.24 (and then some), the 37-year-old has looked better at this year's Wimbledon than she's looked at any major she's played since returning from her maternity leave last year. 

Expectations were tempered for Serena coming into Wimbledon this year, not because anyone doubted her greatness but because there were too many questions about her fitness and form. Finally healthy after healing up a knee injury that has plagued her season, Serena said herself that she was in desperate need of matches. She knew she was hitting the ball well. The question was whether she could do it under pressure. It did not take long to learn that she could. 

Serena looked patchy in her first three sets at Wimbledon, defeating Italian qualifier Giulia Gatto-Monticone in straight sets before dropping the opening set against 18-year-old Slovenian qualifier Kaja Juvan. Since then, she has looked superb. There have been no signs of panic or insecurity since, as she quickly made adjustments to dominate Juvan 2-6, 6-2, 6-4, and then raise her levels to play a sharp match against Goerges in the third round, winning 6-3, 6-4. 

"She's pain-free for three weeks now, so she feels so much lighter," coach Patrick Mouratoglou said. "When you focus on your pain every day because you are in pain, it is very difficult to prepare well for tournaments. You have to adapt everything you do to the pain. When you compete you compete with pain so it's difficult to focus on the match because you are suffering. 

"Now things are different. We could have two weeks of preparation, not thinking about any pain but thinking about what we need to do to get ready. When she's playing she's 100% focusing on the game and winning. That's the most important. She needed time in competition, and with both singles and mixed doubles, she's been able to spend a lot of hours on the tennis court playing matches. That's what she needed." 

But the true test of Serena's match fitness came in the quarterfinals, as Alison Riske brought all of her grass-court sensibilities to Centre Court to push Serena in a tense and tight final set. When it looked like Serena might run away with the decider, Riske pulled her back. Yet, under consistent pressure, Serena never hung her head. She did not look to the sky and wonder why the fates were conspiring against her. She did not shoot a look across the net in frustration that someone dared challenge her. 

No, this time Serena did not look outward for solutions or explanations. This time she looked inward. She dug deep and backed herself to come up with something special to drive herself over the line. The best example came in the final game, as Serena put in a tight double-fault to go to 5-3, 15-all. Serena turned around and muttered to herself, "That's ok. You got this."

That would be the last point Riske would win. 

After firing a beautiful and unexpected sharp-angle running cross-court pass for a winner, Serena turned to her box with a steeled jaw and quiet fist-pump. After firing down her 19th ace on match point to close the match, she again turned to her box, this time with arms raised triumphant, before a roar and double fistpump. Looking back on the entirety of her 13-month comeback, it was the first true sign that Serena Williams, both in mind and aura, is back. 

Just like Serena, No.54 Barbora Strycova has lost just one set en route to the semifinals. The 33-year-old paved her own way through the draw, knocking out four seeds in No.32 Lesia Tsurenko, No.4 Kiki Bertens, No.21 Elise Mertens (where she rallied from a set and 2-5 down), and No.19 Johanna Konta to become the oldest first-time Slam semifinalist in the Open Era. 

Strycova is the feel-good story of the second week. A quarterfinalist here in 2014, Strycova is playing what could be her last Wimbledon. The Czech told reporters earlier in the week that she has no idea if she will retire before the next edition rolls around, but that she wanted to finish her career with no regrets. 

"I want to enjoy it," Strycova said. "Tennis gave me so much good stuff, so many memories, emotions and moments that I want to also give it back. 

"I don’t know if it’s going to be this year or next year the finish, but I want to finish with the respect for the sport."

To make her first major semifinal and set up a fourth career showdown against Serena, Strycova has harnessed her emotions and played her aggressive brand of disruptive grass-court tennis. For nearly every two baseline points she's played she has come to the net once, and she has kept the pressure on her opponents with smart serve and volley plays. When she has faced bigger servers, like Konta and Bertens, she has still managed to make over 86% of her returns each time. 

"If you look at the draw, I don't know how many players can play this way," Strycova's coach Lukas Dlouhy said. "I never saw a player so talented like Barbora for the touch for the ball and covering the court. I think that's why she's playing good on grass. She always loves to be at Wimbledon and she loves the tournament."

But Strycova has never taken a set off Serena in their three prior meetings, all of which have come at the Slams. Their most recent meeting was their tightest, with Serena winning 7-5, 6-4 in the Round of 16 en route to her last major title at the 2017 Australian Open. 

"Now [Barbora] is at the top of her game," Dlouhy said. "Serena is winning everything, but she has days when you can beat her. Hopefully we can find out the way to do it tomorrow."

Strycova's serve tops out at 111mph compared to Serena's 121mph max at the tournament, and the key for the Czech will be fending Serena off in her service games. Strycova has served remarkably well over the fortnight. In fact, she leads the Final Four in service games won at 83%, having been broken 9 times. Serena is second at 80%, having been broken 11 times. 

Unreturned 1st Serves
Serena: 45%
Strycova: 31%

Unreturned 2nd Serves
Serena: 22%
Strycova: 21%

1st serve points won
Strycova: 73%
Serena: 72%

2nd serve points won
Serena: 54%
Strycova: 53%

Break Points Saved
Strycova: 65%
Serena: 27%

"I don't want to talk about it that I'm just in the semifinal," Strycova said. "I am still in the draw. I can still go. I believe in myself. So we see what's going to happen on Thursday."