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Sabalenka has enjoyed a spectacular season, defending her Australian Open title in January and adding the US Open trophy to her cabinet last month, as well as enjoying victories in Cincinnati and Wuhan.
Playing in her first major semifinal, Pegula was made to look like a novice by Muchova in a disastrous opening set, but somehow turned the tables on the unseeded Czech.
Fritz seized back momentum in the third set and closed out the match in a fourth-set tiebreak.
Djokovic, who has not competed since winning the gold medal at the Paris Olympics, will begin against a qualifier and is expected to meet Andrey Rublev in the quarterfinals and fourth seed Alexander Zverev in the semis.
Zverev edged Dutchman Tallon Griekspoor 3-6 6-4 6-2 4-6 7-6 (10-3).
Defending champion Djokovic outclassed Spaniard Roberto Carballes Baena 6-4 6-1 6-2.
Ruud swept aside Brazil's Felipe Alves 6-3, 6-4, 6-3 while Sabalenka mowed down Russia's Erika Andreeva 6-1, 6-2.
The title was the 21st of Swiatek's career as the world No.1 extended her unbeaten run to 12 matches on clay to sound out a warning to her rivals ahead of the French Open.
Victory improved Swiatek's record over her Belarusian rival to 7-3 as the three-time French Open champion bagged the only major claycourt title that was missing on her glittering resume.
After a gutsy second-round win on Friday over good friend Paula Badosa the world No. 2 appeared to have little left in the tank physically, mentally or emotionally for a late night Stadium court battle with Ukrainian Kalinina.
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