Rome: Serbia's Novak Djokovic, the second seed, cruised into the semi-finals of the Rome Masters Saturday after a rain-interrupted 6-1, 6-4 win over Juan del Potro.
Austria's Dominic Thiem, who sent seven-time champion Rafael Nadal crashing out in their quarter-final, is set to face the victor of the Djokovic-Del Potro quarter-final later in the day.
Djokovic was leading 6-1, 1-2 before heavy rain Friday night caused the conclusion of the match to be postponed by a day.
Djokovic waved his arms to get the crowd behind him as the set wore on, dictating play from near the baseline while del Potro stood further back retrieving balls in the corners.
The first men's semi-final is between 20-year-old German starlet Alexander Zverev and big-serving American John Isner.
Simona Halep beat Kiki Bertens, 7-5, 6-1, scoring her 10th straight win to reach her second straight final in Rome.
Williams saved five breaks points in the second before seizing her only chance to take the match into a decider, but Muguruza recovered her composure to see off the American veteran in an hour and 46 minutes. After getting broken in that game she held at love in her next service game to end the set.
Bertens committed 31 unforced errors to Halep's nine.
The classy Halep won the Madrid Open last weekend after reaching the last four of the Stuttgart Open and could claim back-to-back titles in Rome after dispatching qualifier Kontaveit 6-2 6-4.
Ukraine's Elina Svitolina faught off a fightback by Czech favourite Karolina Pliskova to book her semi-final place.