In a battle of the top two seeds, it was the second seed who came out on top, with Ostapenko triumphing 7-6 (8) 6-4.
It was the Latvian’s first title since Dubai last February and her first on grass since Eastbourne in 2021.
Ostapenko clinched a very tight first set on her fourth set point in the tie-break after saving one chance for Krejcikova and moved to the brink of victory at 5-1 in the second set.
Czech Krejcikova, who will return to the top 10 on Monday, threatened a comeback, winning three games in a row, but Ostapenko made it across the line.
“She’s a really great player and congrats to her and her team,” said the 26-year-old, who has survived several close battles this week.
“I was really close from 5-1 to 5-4 but then I managed somehow and I’m really, really happy about it. I was fighting every match. I played five great matches, this was the only match in two sets. It’s a great preparation for Wimbledon and there’s still a couple of things I can do better.”
Krejcikova, who was playing her first final on grass, did not drop a set until the final and felt losing the first-set tie-break was crucial to the outcome.
“She’s playing well,” said the top seed. “I’m disappointed but that’s tennis. (The tie-break) was maybe the key to the match because when you play the first set that long and you are up, down, up, down all the time – I was very unfortunate.
“I definitely had a great week. It was really nice to be here, I really enjoyed it. It’s special to play on a centre court and to enjoy the support. I’m definitely looking forward to coming back.”
Kerber made home advantage count, following up a 2-6 6-3 6-3 quarter-final win over Amanda Anisimova – a match that was pushed back from Thursday when rain washed out play – with a triumph over top seed Petra Kvitova.
The former world number one came back from behind to win 3-6 6-4 7-6 (7-3) and take a spot in her first WTA singles final since 2019.
Kerber and Kvitova have now met 15 times, with the German winning on nine occasions.
She will go up against an unexpected finalist in the form of Siniakova, who defeated Sara Sorribes Tormo.
The Czech is the world number two in doubles, winning three grand slam titles in that format, and will be looking to replicate the feat of her regular partner Barbora Krejcikova, who triumphed at the French Open this year.
At the Viking International in Eastbourne, Jelena Ostapenko made light work of Elena Rybakina, while Anett Kontaveit made it into the final due to Camila Giorgi's retirement.
Latvian wildcard Ostapenko finished her 6-4 6-1 victory with zero double faults – the first time she has managed such a performance since winning in Luxembourg two years ago.
"I'm really happy with the way I played this week, and every match I played better and better," Ostapenko told a news conference after clinching a spot in her ninth career final.
"It's been a while [since making a] final, so I'm really happy to be in a final and looking forward to tomorrow's match."
The 2017 French Open champion will face Estonia's Kontaveit, who has won their previous two meetings and was 5-4 up in the opening set against Giorgi when her opponent retired hurt.
"It's actually nice to have a Baltic final, because [we're] not such big countries, but we still have good players," Ostapenko added.
"Anett, I know since juniors. We have been playing a lot, maybe not so many matches but we practice sometimes together, in juniors, and played even doubles. So I know her quite well."
Champion in St Petersburg two weeks ago, Kontaveit extended her winning run to eight matches following a 6-4 6-1 success.
The opening set was neck and neck until Kontaveit won 15 out of 16 points from 30-30 at 4-4 to draw first blood and establish a 2-0 lead in the second.
The Estonian, who registered just 12 unforced errors throughout the contest, then took 12 of the last 15 points to advance to the last four.
"At times, it was just kind of fun because I felt like the level was really good, and I was playing really well," said Kontaveit, who has won nine of her last 13 matches against top-10 opposition.
"I think I have this confidence that I can go deep every week that I'm playing, and I'm not setting these mental barriers to myself that I used to do.
"I'm very happy with being in the semi-finals, but I also feel like I can still go deeper, I can still do more. So, I'm not too satisfied yet with this result. I feel like if I play well or if I'm consistent, I have a chance with anyone.
"I'm really happy that I'm bringing a good level of tennis consistently every tournament. I think that's something that I'm just most pleased with."
In the semi-finals, Kontaveit will face another in-form player in Jelena Ostapenko, who saw off Garbine Muguruza in straight sets.
Having triumphed in Dubai last week, the 15th seed has now recorded nine successive victories for the first time in her career after prevailing 6-2 6-2.
Ostapenko struck 39 winners and claimed 15 of 18 points on her opponent's second serve as she moved a step closer to a maiden title in Doha, having reached the final six years ago.
Meanwhile, Iga Swiatek is through to her first WTA 1000 semi-final on a hard court after a 6-2 6-3 win over Aryna Sabalenka.
The former French Open champion won seven out of eight breakpoints, while reeling off six successive games in the second set to secure a comfortable victory.
Next up for her is Maria Sakkari, who recorded her third win against Coco Gauff in four meetings after prevailing 6-3 6-3.
The sixth seed is through to her sixth semi-final at WTA 1000 level or above – and targeting her first such title.
Swiatek advanced to the Doha showpiece for the first time after managing her first win over Sakkari in four attempts with a 6-4 6-3 triumph on Friday.
The Pole was in imperious form as she delivered 20 winners to overcome sixth seed Sakkari in an hour and 28 minutes, collecting her seventh top-10 win and second straight such win after defeating Aryna Sabalenka in the quarter-finals.
The seventh seed will now meet Kontaveit, who continued her impressive streak with a 6-1 6-4 win over Ostapenko in just 68 minutes.
Ostapenko headed into the clash on a career-best nine-match winning run, but succumbed to her fourth loss in five meetings with the fourth seed in Qatar.
Ostapenko did have back-to-back break points at 5-5 in the final set, but Kontaveit managed to hold on for victory.
"She can outplay anyone, and then she might not find the consistency sometimes, so you have to be ready for that," Kontaveit said of Ostapenko.
"I was trying to play a consistent match and just do my best on my serve and hold on to it, especially in the second set when I was a break up.
"Sometimes when she's on fire, her returns are unplayable and it's just very difficult.
"But I kept repeating to myself, 'I'm really good at serving it out, I'm really good at serving it out,' and eventually managed to do that.
"How you approach the match, how you're approaching the service games, I think if you're trying to be aggressive [...] that has a lot of effect on the serve."
It will be the seventh final that Kontaveit has contested since last August, converting those appearances into five titles so far. Kontaveit and Swiatek share a pair of wins apiece in previous clashes.
Reigning champion Ostapenko had not dropped a set on the way to the final, but could not halt Kvitova, who was making her first appearance in a final in 2022.
The former world number two – and two-time Wimbledon champion – made a strong start, breaking Ostapenko early on and racing to a 3-0 lead in the first set.
Ostapenko, the Latvian number eight seed, faced eight break points in the opening set, saving seven, but she was unable to force any of her own as Kvitova comfortably served out to move ahead.
Kvitova was hitting the ball with immense power, particularly on returns, but Ostapenko showed initial improvement in the second set with her first serve accuracy, which had been down at 55.9 per cent in the first.
However, it was not enough to keep her opponent at bay as some more fierce returning from Kvitova saw her break in the third game of the second set.
Ostapenko finally threatened to break the Czech's serve, but was unable to take any of the five break points she earned in a game that lasted more than 12 minutes.
Her first serve dropped off again, which allowed the relentless Kvitova to take full advantage, breaking for a second time before serving for the championship and sealing her first-ever Eastbourne title.
It was Kvitova's 29th triumph on the WTA Tour, but her first since March 2021. Her last success on grass had come in Birmingham four years ago.
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.
World number 26 Camila Giorgi battled from a break down in both sets to beat fifth seed Muguruza 7-5 6-3 in an hour and 53 minutes.
That meant 12th seed Giorgi made her second straight quarter-final, following a last-eight appearance in Birmingham, and will next face Viktoriya Tomova, who defeated Kirsten Flipkens 3-6 6-3 6-4.
Jil Teichmann, the 10th seed at the tournament, was a notable second-round elimination, falling to a 7-6 (9-7) 4-6 6-3 defeat to Briton Harriet Dart, who later triumphed 6-4 2-6 6-4 over Marta Kostyuk.
Dart's fellow Briton Katie Boulter stunned last year's Wimbledon runner-up Karolina Pliskova in the second round but had no such luck against Petra Kvitova, losing 5-7 6-0 7-5.
Beatriz Haddad Maia, the winner at the Birmingham Open last week, extended to a 12-match winning streak with a 6-1 6-2 victory over Jodie Burrage.
Jelena Ostapenko also progressed after Madison Keys retired when 6-3 down and will next face Anhelina Kalinina, who battled to a 6-3 2-6 6-3 win over 16th seed Yulia Putintseva.
While seeds fell at Eastbourne, there were not as many shocks at the Bad Homburg Open, where Angelique Kerber defeated Lucia Bronzetti 6-2 6-3 to reach the quarter-finals.
Fourth seed Simona Halep also made the last eight with 6-0 6-3 victory over Tamara Zidansek and will meet Amanda Anisimova after she won an All-American match against Ann Li 6-0 6-2.
Alize Cornet downed Tatjana Maria 7-6 (7-4) 6-4 to tee up a clash with Kerber, while Bianca Andreescu will meet top seed Daria Kasatkina after defeating Katie Swan 6-4 6-4.
Bencic, the unlikely successor in Tokyo, triumphed in straight sets 6-1 6-3 on Thursday to tee up a last-eight encounter with Liudmila Samsonova, who the Swiss lost to in Berlin earlier this year.
It was a welcome return to form for world number 12 Bencic after her defeat to eventual champion Emma Raducanu in the US Open quarter-finals last week.
Joining the top seed in the quarters is defending Luxembourg Open champion Jelena Ostapenko, who needed three sets to overcome Arianne Hartono.
Second seed Elise Mertens also progressed, though she had to come from behind to beat Aliaksandra Sasnovich 3-6 6-2 7-5.
In Thursday's other last-16 tie, Marie Bouzkova beat Greet Minnen to round off the quarter-finals line up.
Meanwhile, Sorana Cirstea and Jasmine Paolini will meet in the Zavarovalnica Sava Portoroz quarter-finals after respective victories over Tereza Martincova and Anna Kalinskaya.
Four-time grand slam champion Osaka won the opening game, before Saville was unable to continue due to a scary-looking injury to her left knee.
The former world number one will now face a second round match with world number 16 Beatriz Haddad Maia, who eased to a straight sets victory over Yuki Naito.
Karolina Pliskova secured her place in the next round in Tokyo with a 6-2 6-1 demolition of Isabella Shinikova, while Zhang Shuai will face second seed Caroline Garcia on Wednesday after overcoming Mai Hontama in two sets.
In Seoul, Jelena Ostapenko's hopes of repeating her 2017 heroics at the Korea Open are still alive after the number one seed narrowly beat 19-year-old Jeong Bo-young.
The world number 19 looked to be cruising after the opening set, but the unheralded South Korean hit back in the second and took it to a tie-break in the third, before Ostapenko's class showed as she won the crucial tie-break 7-2.
Elsewhere, 2021's beaten finalist Kristina Mladenovic saw off wildcard Park So-hyun 6-4 4-6 6-3, while second seed Ekaterina Alexandrova overcame a worrying second set to beat Asia Muhammad 6-1 3-6 6-1.
Wimbledon semi-finalist Tatjana Maria is also safely through to the second round after a tight 7-6 (7-5) 7-6 (7-5) triumph over Eugenie Bouchard.
The former world number one, who won the Tokyo tournament when it was last held in 2019, was unable to compete in Thursday's match due to illness.
"I am really sorry that I am not able to compete today," Osaka said. "It's an honour to be able to play at the Pan Pacific Open in front of the amazing fans here in Japan.
"This has and always will be a special tournament for me and I wish I could have stepped on court today, but my body won't let me."
Haddad Maia advanced on a walkover and will next face fourth seed Veronika Kudermetova, who defeated Mexican qualifier Fernanda Contreras Gomez 6-0 6-1 to reach the quarter-finals.
Liudmila Samsonova also progressed to the last eight, with a 7-6 (7-5) 6-3 win over Wang Xinyu continuing her good form.
However, sixth seed Karolina Pliskova exited the competition on Thursday at the hands of Petra Martic, who prevailed 6-3 6-4 and will face Zhang Shuai next.
At the Korea Open, top seed Jelena Ostapenko recovered from two games down in the deciding set to overcome Anastasia Gasanova 6-3 5-7 7-5.
Ostapenko remains on a possible semi-final collision course with Emma Raducanu, who came through 6-4 7-5 against Yanina Wickmayer, while Magda Linette saved two match points to beat Kristina Mladenovic 4-6 7-6 (7-5) 6-2.
Ostapenko, looking to add to the fifth career title she won in Dubai 12 months ago, edged an entertaining contest 7-5 1-6 7-5 in two hours and 19 minutes.
Fellow seeded player Anett Kontaveit was also victorious on the opening day of the tournament, but only after battling back to beat Zhang Shuai 4-6 7-6 (7-4) 6-4.
Marta Kostyuk saw off Sorana Cirstea 6-2 1-6 6-2, meanwhile, and will now face second seed Belinda Bencic in the next round.
Elise Mertens is also through to round two, where Veronika Kudermetova awaits, after dominating Martina Trevisan 6-0 6-2.
In the first main draw match at the Linz Open, Sofia Kenin let an early lead slip to lose 2-6 6-3 6-4 to Jule Niemeier.
Ostapenko looked to be cruising after winning the opener, but Kanepi levelled in the second despite the Estonian watching five set points come and go.
Kanepi then romped to victory with a stunning final set to defeat Ostapenko 4-6 6-4 6-0 and keep her hopes of winning her home tournament alive.
In second-round action, Beatriz Haddad Maia is through to the quarter-finals after a 6-1 7-5 success over Linda Noskova.
After Haddad Maia cruised to a one-set lead, the second was a much tighter affair before the Brazilian's class showed as she broke Noskova and then held serve to earn a straight-sets win.
Second seed Belinda Bencic also secured her place in the final eight with a 6-4 6-7 (2-7) 6-3 triumph over Katie Boulter.
At the Parma Ladies Open, top seed Maria Sakkari came from behind to beat Arantxa Rus and seal her quarter-final spot.
Rus saved five out of five break points as she won the opening set 6-3 to give her hope of an upset.
Those hopes were soon dashed though as world number seven Sakkari broke twice to take the second set 6-2, before clinching the match in the third to tee up a clash with Maryna Zanevska.
World number 33 Irina-Camelia Begu made it 12 consecutive wins on clay by overcoming Matilde Paoletti in straight sets and will face fellow Romanian Ana Bogdan in the next round after she beat Anna Karolina Schmiedlova 6-2 3-6 6-3.
Fourth seed Sloane Stephens saw her tournament come to an end after she was defeated by Danka Kovinic.
Ostapenko needed less than an hour to get past Paula Badosa, who caused an upset when the pair met at last year's delayed French Open.
There was to be no Roland Garros repeat for Badosa, though, as she was swept aside 6-2 6-2.
The sixth-seeded Ostapenko started her campaign impressively in Russia, producing 26 winners and just eight unforced errors. Her serve was also in good order, never giving her opponent a chance of a break.
Sasnovich had to work a little harder, rallying from a set down to see off Ana Bogdan 2-6 6-2 6-1. Her reward is a clash with third seed Fiona Ferro, who received a bye through to the second round.
Zvonareva, meanwhile, progressed in straight sets against Arina Rodionova, the Russian dropping just four games during a contest that lasted one hour and 22 minutes.
Victory was sealed at the second opportunity, a sliced backhand into the net by her opponent sending Zvonareva through to the last 32.
Ostapenko, who is the world number 21, stunned Halep in their first meeting in the 2017 Roland Garros final to win her first Grand Slam before the Romanian exacted revenge in Beijing later that season.
In their first meeting since that last-four clash, Halep raced out the blocks to wrap the first set in just 27 minutes as she converted both break-point opportunities to take an early lead.
However, Ostapenko responded emphatically in the following set, claiming a 3-0 advantage before eventually levelling things up after a one-sided tie-break separated the pair in the second.
Ostapenko carried her momentum in the deciding set as she made a blistering start, with Halep unable to win a single game as the 24-year-old secured victory in an hour and 36 minutes.
Kudermetova awaits the Latvian in the final after Marketa Vondrousova was forced to withdraw from her last-four match due to a right adductor injury.
The pair have not met before on the WTA Tour but boast identical records at the start of the 2022 season, with both winning eight of their matches and losing three.
Ostapenko eased to a 6-0 6-4 win in just 65 minutes, which is expected to take her up to number 13 in the rankings, the first time the former world number five will have been in the top 20 since October 2018.
An impressive week for the Latvian has seen her beat Sofia Kenin, Iga Swiatek, Petra Kvitova, and Simona Halep on her way to the final, and she never looked like slipping up here.
It was consistency that made the difference, with Ostapenko winning 63.6 per cent of points on her first serve, but also an impressive 58.3 per cent on her second.
She took the first set in just 23 minutes, with her serve allowing her to zip through games with ease as Kudermetova had no answer to it, or indeed her returns.
The Russian recovered well in the second set and earned an early break, but things soon swung back to take Ostapenko to 5-3 and serving for the title. Kudermetova managed to break, only for her opponent to do the same to seal the win.
Ostapenko won as a wildcard in 2021, and the world number 14 confirmed her place in this year's showdown by overcoming Camila Giorgi on Friday.
She is the first female player to reach back-to-back finals in Eastbourne since Caroline Wozniacki in 2017 and 2018.
The Latvian, who is also going to compete for the doubles title, prevailed 6-2 6-2 and will now go up against Kvitova. The pair have faced off eight times previously, with each player winning four matches.
It is Kvitova's first appearance in a final in 2022, with the former world number two – and two-time Wimbledon champion – having ended Beatriz Haddad Maia's winning streak.
Haddad Maia won in Birmingham last week and Nottingham the week before, but her run came to an end at 12 matches, with Kvitova triumphing 7-6 (7-5) 6-4.
"For me, a final after almost a year [without one] will be great, so I’m glad already," said Kvitova, who was a runner-up at Eastbourne in 2011 and last reached a final on grass in Birmingham four years ago.
"Jelena loves to play here, obviously, we saw it, she has a really great game for grass."
Meanwhile, at the Bad Homburg Open, 2019 US Open champion Bianca Andreescu reached her sixth career final courtesy of a walkover against Simona Halep.
The Romanian withdrew from the semi-final clash with a neck injury, which will worry the 2019 Wimbledon champion ahead of the season's third grand slam at the All England Club.
"I am sorry that I had to withdraw today before my semi-final match," Halep said in a statement.
"But unfortunately I woke up this morning with a blocked neck and this is not allowing me to perform to the best of my ability."
Andreescu will face Caroline Garcia, who saved a match point before going on to beat fellow Frenchwoman Alize Cornet 7-6 (11-9) 3-6 7-5 to reach an 11th tour-level showpiece.
The Latvian was the first wildcard to reach the final since Daniela Hantuchova in 2000 and kept up her good form ahead of Wimbledon by beating Kontaveit 6-3 6-3.
French Open 2017 winner Ostapenko dominated from the start and picked up the victory in one hour and five minutes, adding to her three previous career WTA titles.
She has now won titles on all four surfaces – clay, hard, indoor and grass – and is hopeful of improving on her current ranking of 43rd in the world.
"I think it says that I can be dangerous and playing well on all surfaces probably," she said in her on-court interview.
"Of course I enjoy to play more on just some of them, but in general I can adjust well to different surfaces.
"I think this is just the beginning for me. If I keep playing the way I played this tournament, I think I can be back in top 10 and play well.
"I just have to keep that in my mind and work even harder."
Ostapenko made good use of her strong service game to break Kontaveit's serve in the third and fifth games.
Kontaveit earned a first break in the eighth game with four service return winners in a row, but Ostapenko recovered from 40-15 in the next game to take the opening set.
It was then plain sailing for Ostapenko in the second set as she earned the only break at 2-1 and held for the remainder, clinching the victory with her second championship point.
Ostapenko will face Leylah Fernandez in the first round of Wimbledon next week.
Two-time grand slam champion Kvitova was comfortably beaten in straight sets by Irina-Camelia Begu, with the Romanian winning 6-4 6-0 in just one hour and nine minutes.
The effectiveness of the first serve was the main difference-maker, with Begu winning 73.1 per cent of her first-serve points, while Kvitova – who has 28 singles titles to her name in contrast to her opponents' four – could only manage 50 per cent.
Begu was set to face the winner of Tereza Martincova's tie with Elena Rybakina, but the latter withdrew because of illness.
Seventh seed Ostapenko made light work of Andrea Petkovic as she beat the German 6-1 6-2 in just 58 minutes, helped largely by winning 85.3 per cent of her first serves, as well as saving all five of the break points she offered up.
Ostapenko has now won two of her four meetings with Petkovic and will next face Aliaksandra Sasnovich, who eased to a 6-2 6-3 victory over Jaqueline Cristian.
Jelena Ostapenko will face either Ons Jabeur or wild card Sofia Kenin in the Qatar Open quarter-finals after dispatching Jasmine Paolini
Potapova won the first set in 39 minutes, claiming the only break of the frame in the sixth game, but Pegula responded by breaking immediately in the second.
Despite squaring the match up, the American trailed 3-1 in the third set, only to fight back again and triumph in two hours and 17 minutes.
Pegula will face 15th seed Petra Kvitova after she won a seesawing three-set contest over 24th seed Jelena Ostapenko, 0-6 6-0 6-4.
Ostapenko won the first six games, before Kvitova won the next 10, only for the Latvian to hit back and claim the next four, squaring up the deciding set at 4-4. But two-time Wimbledon champion Kvitova held her nerve and won the final two games for victory.
World number two Aryna Sabalenka progressed to the final 16 via walkover after her third-round opponent Lesia Tsurenko withdrew.
The 2023 Australian Open champion will take on 2021 French Open winner Barbora Krejcikova after she toppled Wang Xin 6-2 6-7 (1-7) 6-2.
Seventh seed Maria Sakkari secured victory in a two-and-a-half-hour third-round clash with Anhelina Kalinina, winning 3-6 6-2 6-4.
Two-time major runner-up Karolina Pliskova won 6-1 7-5 over Veronika Kudermetova, progressing into the last eight to face Sakkari.
Sixth seed Coco Gauff, who turns 19 on Monday, eased past 54th-ranked fellow teenager Linda Noskova 6-4 6-3 in one hour and 19 minutes.
Sweden's Rebecca Peterson continued her resurgent form with a 3-6 6-3 6-1 win over Jil Teichman, setting up a clash with Gauff.
