World number one Serena Williams crushed Italian Sara Errani 6-1 6-3 to reach the semi-finals of the French Open in intimidating fashion.

American Williams, a two-time winner at Roland Garros, hammered 39 winners in a one-sided encounter against Errani, who was a runners-up here in 2012.

Williams had dropped the opening set in the previous three rounds but made no such slow start on Wednesday, setting the tone by breaking in Errani's first service game.

A brilliant backhand passing shot handed Errani an immediate break back but Williams cantered to the opening set.

Errani upped her pace in the second as she looked quicker on her feet, but it was still not enough as Williams broke for a 4-3 lead and she finished it off on her fourth match point.

"I definitely felt more in control, and I felt I had to be again, going up against a player who plays well on this surface in particular," said Williams, who has only been in three previous semi-finals at the French Open.

The American, however, says she is not feeling the strain of chasing her 20th major singles title.

"I'm not putting too much pressure on myself... I'm surprised I'm still in the tournament, so this is really cool for me."

Williams, who is seeking a 20th grand slam title, will take on either Timea Bacsinszky of Switzerland in the final.

Bacsinszky toppled little-known Belgian Alison van Uytvanck 6-4, 7-5 in the most unexpected of French Open quarter-finals.

Bacsinszky, seeded 23, produced a fearless brand of baseline tennis to became the first Swiss woman since Martina Hingis in 2001 to reach the last-four at Roland Garros.

Bacsinszky became the first Swiss woman since Hingis in 2001 to reach the French Open semi-finals [Getty Images]

Source: Al Jazeera and agencies