Doha - Sofia Kenin suffered her third defeat since winning the Australian Open earlier this month, losing to Ukrainian youngster Dayana Yastremska in the Doha second round, while world number one Ashleigh Barty cruised to victory on Tuesday.
American world number five Kenin, who defeated Garbine Muguruza to claim her maiden Grand Slam singles title in Melbourne, went down 6-3, 7-6 (7/4) to Yastremska.
Kenin lost one of her two matches in the United States' Fed Cup qualifying win over Latvia, before being knocked out by in-form Kazakh Elena Rybakina in the Dubai opening round last week.
"It's really frustrating," the 21-year-old told wtatennis.com.
"Obviously coming off of Melbourne where I felt I was playing the best tennis of my life, coming down to playing, not the worst tennis, but not playing the tennis I want to be playing."
The 19-year-old Yastremska will face either Muguruza or Australian Ajla Tomljanovic in the third round.
"Creds to her, she played well, came up with some good shots at those important moments," Kenin said of Yastremska, who has already won three WTA titles.
"She just played better at important moments and I just made too many unforced errors, I just felt pretty flat-footed the whole match."
Barty made short work of German Laura Siegemund in her first match since losing in the Australian Open semi-finals to Kenin.
The reigning French Open champion saw off the world number 68, 6-3, 6-2.
Barty broke serve five times in the match, winning in under an hour and a quarter.
The Australian could next face Rybakina, who has already reached four WTA finals this season and plays Alison Van Uytvanck in her second-round encounter.
Two-time Wimbledon champion Petra Kvitova also booked a place in the last 16 with a 4-6, 6-3, 6-0 win over Spaniard Carla Suarez Navarro.
The Czech will face former Roland Garros winner Jelena Ostapenko for a quarter-final spot after the Latvian's 6-1, 6-2 thrashing of Barbora Strycova.
Last year's winner Elise Mertens saw her title defence come to an early end with a 4-6, 6-3, 6-2 loss to Yulia Putintseva.
Kazakhstan's Putintseva will next play fourth seed Belina Bencic after the Swiss battled past Veronika Kudermetova 6-4, 4-6, 7-6 (7/4).
Belarusian ninth seed Aryna Sabalenka also progressed, beating Anett Kontaveit 7-5, 2-6, 7-5.
Later on Tuesday, world number three Karolina Pliskova faces American Bernarda Pera.
Results from the WTA tournament in Doha on Tuesday (x denotes seeding):
Ashleigh Barty (AUS x1) bt Laura Siegemund (GER) 6-3, 6-2
Dayana Yastremska (UKR) bt Sofia Kenin (USA x6) 6-3, 7-6 (7/4)
Jelena Ostapenko (LAT) bt Barbora Strycova (CZE) 6-1, 6-2
Petra Kvitova (CZE x8) bt Carla Suarez Navarro (ESP) 4-6, 6-3, 6-0
Svetlana Kuznetsova (RUS) bt Iga Swiatek (POL) 6-2, 6-2
Yulia Putintseva (KAZ) bt Elise Mertens (BEL x16) 4-6, 6-3, 6-2
Zheng Saisai (CHN) bt Vera Zvonareva (RUS) 7-5, 6-1
Aryna Sabalenka (BLR x9) bt Anett Kontaveit (EST) 7-5, 2-6, 7-5