American Danielle Collins powered into the semi-finals of the WTA Miami Open with an emphatic 6-3, 6-2 win over France's Caroline Garcia on Wednesday, while world number three Jannik Sinner reached the semi-finals in the men's draw.

Garcia went into the match on the back of an upset win over third-seeded American Coco Gauff but was unable to get a foothold against the Floridian.

Garcia still appeared to be troubled by an injury to her right shoulder and received some treatment early in the second set.

Collins broke to go 5-3 up in the first set and then held serve for the set, then in the second set broke in the third game and never looked back as she wrapped up the win in one hour and 19 minutes.

Garcia said she had felt the pace of back-to-back tournaments.

"It's been a long two weeks for the body for me, between the back at the beginning and then the she shoulder which came back," she told reporters, adding that the shoulder issue was a recurrence of a previous injury.

"I think it's nothing too serious but [shoulder] is always quite problematic for a tennis player," she said, before adding that Collins had played "an amazing match".

Collins intends to retire at the end of the year but while she agreed she had a relaxed look to her play, she rejected the idea that it was a result of her impending departure from the sport.

World number three Jannik Sinner moved into the semi-finals in emphatic style, beating Czech Tomas Machac 6-4, 6-2.

Italian Sinner, who won his first Grand Slam title at the Australian Open this year, will play in his fourth semi of the season after dealing with lively Machac in 91 minutes.

Sinner broke the opening game serve from the Czech but Machac responded immediately to make it 1-1.

The crowd were on their feet to appreciate some brilliant defensive play from Sinner who eventually took charge of the set, breaking to go 4-3 up and then closing out.

The second set was more straightforward with Sinner breaking in the third game and never looking back. (AFP)