A resolute Serena Williams recovered from a set and a break down to beat Tsvetana Pironkova on Wednesday and return to the US Open semi-finals.

Williams' pursuit of a long-awaited record-equalling 24th major title appeared in serious trouble as she struggled to effectively combat the resurgent Pironkova's combination of forehand slice and backhand power.

The Bulgarian - a 2010 Wimbledon semi-finalist - had not appeared on the WTA Tour since playing the All England Club three years ago but dumped out two seeds en route to this stage.

As the finish line quickly moved into sight on Arthur Ashe, though, the great Williams proved a step too far for Pironkova.

The 38-year-old, beaten in the previous two Flushing Meadows finals, dug in for an exhausting 4-6 6-3 6-2 success and set up her latest semi against either Elise Mertens or Victoria Azarenka.