American teenager Coco Gauff suffered a shock defeat by Katerina Siniakova in the second round at the Qatar Open.

World number three Gauff lost six games in a row in the second set and was beaten 6-2 6-4 by Czech Siniakova.

The 19-year-old Gauff was making her first appearance since an Australian Open semi-final loss to Aryna Sabalenka last month.

Despite being seeded second in Doha, Gauff could not find her usual rhythm against 27-year-old Siniakova.

After losing the opening set, last year's US Open champion regained her composure to race into a 4-0 lead in the second set.

But Siniakova, who is a seven-time Grand Slam champion in women's doubles, hit back spectacularly to claim one of the biggest victories of her singles career.

Also on Tuesday, Naomi Osaka booked her place in the last 16 with a straight-set victory against Croatia's Petra Martic.

Four-time Grand Slam champion Osaka, who is returning to the sport after giving birth to her first child in July, won her second-round match 6-3 7-6 (11-9).

Unseeded Osaka, 26, missed a chance in the second set to take the match at 5-4 and then found herself 4-0 down at the start of the tie-break.

But the Japanese player rallied to save four set points before 33-year-old Martic double-faulted on match point.

Elsewhere in the second round, fourth seed Ons Jabeur was defeated 6-3 6-2 by Ukraine's Lesia Tsurenko, while Australian Open runner-up Zheng Qinwen of China beat Poland's Magda Linette 6-2 2-6 6-3.