Thursday's second-round matches at the WTA Swedish Open were marred by the withdrawal of world number one Serena Williams, who pulled out of her clash with Klara Koukalova in the warm-up due to an elbow injury.

Williams went into the competition in stunning form, clinching her second 'Serena Slam' – holding all four grand slams at once – with a sixth Wimbledon title on Saturday by defeating the impressive Spaniard Garbine Muguruza in straight sets.

Her rivals in Bastad this week could have been forgiven for breathing a collective sigh of relief when Williams retired before even getting underway against Koukalova.

The American revealed a persistent elbow problem had flared up once again and she was not prepared to risk further damage, giving the Czech Koukalova a free passage to the quarter-finals.

As a result of Williams' withdrawal, Barbora Strycova was the next highest-ranked player (37) in action on Thursday and the third seed had little difficulty against Evgeniya Rodina of Russia, winning 6-4 6-1.

Although Strycova lost her serve in the very first game, she showed admirable mettle to fight back, before blitzing the Russian in the second set.

Yulia Putintseva also progressed to the last eight with a 6-0 1-6 6-2 victory over Alize Lim.

Putintseva was irresistible in the first set, winning to love as her progression against the 253rd ranked France native seemed a formality, but the 25-year-old showed great character to break four times in the second set and level the match. 

However, Putintseva rose her game again, with back-to-back breaks at the start of the decider putting her in command, and duly sealed a quarter-final clash with Koukalova.

Sixth-seed Katerina Siniakova was sent packing by Rebecca Peterson 7-5 7-6 (8-6) in a tight encounter, before Johanna Larsson defeated Anna-Lena Friedsam 6-3 7-5.

They will meet Strycova and Mona Barthel respectively in the last eight.