The Latvian was initially due to meet five-time grand slam champion Maria Sharapova in her opening clash at the Lindner Family Tennis Center.

However, the Russian withdrew because of an arm injury to hand Ostapenko a meeting with Serbian qualifier Aleksandra Krunić instead.

The 20-year, the French Open winner on the clay this year, was beaten as she went down 6-4, 6-2.

A mixture of first and second round matches took place today.

American Venus Williams, a seven-time grand slam winner and the Wimbledon finalist last month, progressed from the opening stage with a 6-2, 6-0 win over compatriot Alison Riske.

In round two, Spain's Garbiñe Muguruza, the Wimbledon champion who beat Williams in this year's final, eased past Brazil's Beatriz Haddad Maia 6-2, 6-0 to book her place in the last 16.

Romania's second seed Simona Halep also progressed to round three, beating American Taylor Townsend, the conqueror of Olympic champion Monica Puig, 6-4, 6-1.

Britain's Johanna Konta, a semi-finalist at Wimbledon last month, also made it through her second round tie.

She saw off Kiki Bertens of The Netherlands 6-3, 6-3.

In the men's draw, round one and two matches also took centre stage.

The heavyweight opening round match pitted Argentina's former US Open champion and Olympic silver medallist Juan Martín del Potro against Czech player Tomáš Berdych, who has reached five Grand Slam semi-finals.

It was the Argentine who came through in three sets, 3-6, 7-6, 6-0.

In round two, Austria's third seed Dominic Thiem eased past Italy's Fabio Fognini, 6-3, 6-2.

Croatia's Ivo Karlović also advanced with a victory against France's eighth seed Jo-Wilfried Tsonga, 6-4, 7-6.

The tournament, an Association of Tennis Professionals World Tour Masters 1000 event for men and a Women's Tennis Association Premier 5 competition for women, continues tomorrow.

Spain's Rafael Nadal, who opens in round two tomorrow against France's Richard Gasquet, is set to overtake the injured Sir Andy Murray as men's world number one. 

It is a key build-up event for the last Grand Slam of the season, the US Open, which begins in New York City on August 28.