World number one Iga Swiatek thrashed Coco Gauff to clinch a third straight WTA Tour semi-finals berth after moving into the San Diego Open final four on Friday.

Swiatek was irrepressible against the 18-year-old American, extending her head-to-head dominance to 4-0 with a 6-0 6-3 victory in 66 minutes.

The Pole won the first eight games of the match and always appeared in control with her reliable forehand a key feature.

Swiatek laced 13 winners with only nine unforced errors, breaking Gauff five times. The American committed 26 unforced errors for the match.

The victory is Swiatek's 62nd of the season, moving her closer to Angelique Kerber's 2016 mark of 63.

Swiatek will face fourth seed Jessica Pegula in the semi-finals as the Pole aims for her eighth WTA title of the season.

Pegula secured her berth in the final four with a 6-4 7-5 victory over compatriot Madison Keys in their first-ever meeting in one hour and 34 minutes.

Third seed Aryna Sabalenka was a surprise casualty as Croatian qualifier Donna Vekic continued her excellent week with a 6-4 6-7 (5-7) 6-1 in two hours and 37 minutes.

World number 77 Vekic has beaten former world number three Maria Sakkari and two-time major finalist Karolina Pliskova this week.

Second seed Paula Badosa was also bundled out by American Danielle Collins 7-6 (7-5) 6-4. Collins will meet Vekic in the last four.