Serena Williams struggled but advanced closer to the first calendar Grand Slam since 1988 by defeating Dutch qualifier Kiki Bertens 7-6 (7/5) 6-3 at the US Open on Wednesday.

World number one Williams, a three-time defending champion seeking her seventh US Open title overall, was outplayed early by her 110th-ranked rival but overcame 26 unforced errors and eight double faults to win the first set and found enough form to sustain her march toward history at Arthur Ashe Stadium.

The 33-year-old American is trying to complete the first calendar Grand Slam since Steffi Graf in 1988 and equal Graf’s Open Era record of 22 Slam singles titles, two shy of Australian Margaret Court’s all-time record.

She will face fellow American Bethanie Mattek-Sands for a place in the last-16.