France's Gael Monfils suffered an injury scare ahead of next week's Australian Open when he was forced to withdraw from his Adelaide International Two second-round clash against Brazilian Thiago Monteiro on Wednesday.

Monfils, who won the Adelaide International One tournament last week, took the first set in a tiebreak but appeared to injure his neck at the start of the second set and called for the physio to come to the court.

Monteiro won the second set then broke Monfils' serve in the first game of the third before the top-seeded Monfils decided to withdraw.

The Brazilian won the match 6-7 (2/7), 6-3 1-0 ret to move into the quarter-finals, where he will play French qualifier Corentin Moutet.

Moutet ousted eighth-seeded Hungarian Martin Fucsovics 6-2, 6-2.

In another upset, Australian wildcard Thanasi Kokkinakis saw off second seed John Isner in three tiebreak sets 6-7 (5/7), 7-6 (7/5), 7-6 (7/4).

Russia's Karen Khachanov, who was runner up to Monfils last week, moved into the last eight with a 7-5, 6-3 win over Italian Gianluca Mager.

Former US Open champion Marin Cilic also progressed with a 7-6 (8/6), 6-2 win over Spaniard Jaume Munar.

In the women's draw, 17-year-old Coco Gauff reached the last eight with a two-hour 6-3, 5-7, 6-3 win over fellow teenager Marta Kostyuk.

The third-seeded Gauff is now the highest seed remaining and will be favourite to win her third title on the WTA tour.

She takes on Croatia's Ana Konjuh in the quarter-finals.

Gauff's biggest challenge could come from former US Open finalist Madison Keys, who thrashed Tereza Martincova 6-2, 6-3.

Keys faces eighth seed Liudmila Samsonova in the last eight and could find herself up against Gauff in the semi-finals.