Serena Williams came from a set down to beat Tsvetana Pironkova and reach the semi-finals of the US Open on Wednesday.

The American third seed overcame the unseeded Bulgarian 4-6, 6-3, 6-2 in two hours and 12 minutes inside Arthur Ashe Stadium but it was her third straight three-setter as she struggled to get any rhythm going.

A sluggish start to the match almost saw her hopes dashed as a more energetic Pironkova broke Williams in game five to take an early lead. The unranked player was leading by a set and a break and just when it looked to be all over, Williams mounted a battling comeback.

The 38-year-old worked hard to bounce back and take the second set and looked more comfortable in the third as she reached her third straight and 14th US Open semi-final. She had earlier needed three sets to get past Sloane Stephens and Maria Sakkari as well.

Williams smashed 20 aces, taking her tally for the tournament to 64 so far. The record for the most number of aces in a US Open in the women’s singles is 70, set by Serena helserlf in 1999 when she won the title for first time.

She will play fellow former world No 1 and another tennis mom Victoria Azarenka for a place in Saturday’s final. The unseeded player reached her first Grand Slam semi-final since 2013.

Here’s a look at the big numbers and how Twitter reacted to the best moments from the quarter-final.