Madison Keys reaches Sydney International semi-finals as Bethanie Mattek-Sands retires

Madison Keys has made it through to the semi-finals of the Sydney International after Bethanie Mattek-Sands was forced to retire in the first set.

Eighteen-year-old Keys started strongly on the court by nabbing the first break of serve and taking a 3-1 lead in the opener.

However, Mattek-Sands showed signs of struggle when she called for treatment on her lower back in the early stages of the contest.

It wasn't long before the 28-year-old, who got a game back, decided to call time on her campaign in Australia, resulting in Keys receiving a walkover into the final four.