A dominant Serena Williams proved far too strong for defending champion Simona Halep as the American eased into the Indian Wells tennis semi-finals in California on Wednesday with a 6-4 6-3 victory.

The world No.1 is yet to drop a set in four matches, Williams making a tricky opponent on paper look over-matched in the quarter-final encounter.

The 21-time grand slam champion blasted 28 winners to Halep’s nine. She also recorded seven aces, aptly closing out the match with an unreturnable serve down the middle.

Williams also broke the Romanian’s serve four times and improved her head-to-head record to 7-1 against her fifth-seeded opponent.

A right knee injury forced Williams to withdraw from the event last year, when she ended a self-imposed 14-year absence at the tournament, giving Halep a walkover in their scheduled semi-final.

Halep ultimately defeated Jelena Jankovic to capture the crown but her defeat to Williams kept the American on course for a first Indian Wells triumph since 2001.

Third seed Agnieska Radwanska stands in the way of a spot in the final and Williams expects a tough challenge from the Pole.

“(Radwanska) knows how to play in big stadiums and big matches, big games,” Williams told reporters.

“Obviously, she does everything so well, including running, and this is a good surface for her because it’s a slow surface. I think I expect there will be a lot of long rallies.”

Radwanska booked her semi-final berth when she fought back from 5-2 down in the second set for a 6-2 7-6 (7-3) win over world No.9 Petra Kvitova.

The 27-year-old, who was a runner-up at the event two years ago, raced to a 4-0 lead in the tiebreaker before wrapping up victory after a little more than an hour-and-a-half.

“I am just so happy to be in the semi-finals here,” said Radwanska, set to rise to No.2 in the next world rankings.

“I have nothing to lose, will have a good rest and be ready for the semis.”

Radwanska, who beat Jankovic in the previous round, breezed through the opening set against an error-prone Kvitova on a baking afternoon in the California desert.

Though the Czech produced much better form to open a three-game lead in the second, Radwanska clawed back the deficit before dominating the tiebreaker, helped by a few generous net cords.