Hello
Alcaraz became the first man to defend a championship in the California desert since Novak Djokovic won three straight between 2014 and 2016.
Sakkari got the better of US Open champion Coco Gauff in a hard-fought second semifinal.
Third seed Coco Gauff served up 17 double faults but still managed to beat China's Yuan Yue 6-4, 6-3 to reach the women's semifinals in the Californian desert for the first time.
Four-time Grand Slam champion Swiatek has an excellent record against lower-ranked players but really had to dig deep into her reserves to overcome Collins in a dramatic but scrappy three-hour contest on Rod Laver Arena.
Top-ranked Iga Swiatek faces a tricky test against 2020 champion Sofia Kenin first up in the women's section.
The Pole broke Pegula five times and won the final 11 games in a 59-minute slaughter to capture her first WTA Finals title in a match delayed by a day due to rain
All four players in the group can reach the semifinals.
Swiatek completed the first set comeback on a double fault and a disillusioned Vondrousova went quietly in the second set.
The 2017 Roland Garros champion next faces American sixth seed Coco Gauff, who beat wildcard Caroline Wozniacki in three sets earlier in the day.
Weronika Falkowska and Katarzyna Piter lost 6-4, 6-4 to Heather Watson and Yanina Wickmayer in the doubles final.