Billy Harris exits Hall of Fame Open, Rafael Nadal makes doubles progress

The 29-year-old - who reached the quarter-finals on grass at both Queen's and Eastbourne - had come through qualifying and then beat Canada's Gabriel Diallo in the first round in Newport, Rhode Island.

But the British number four found the going tough against McDonald, who is ranked just inside the top 100.

McDonald broke Harris twice to take the opening set in 34 minutes. Although Harris managed to save two match points against his serve, the American eventually closed out a 6-3 6-4 victory to reach the quarter-finals.

Reilly Opelka pulled off a shock result as he beat top seed Adrian Mannarino 6-4 3-6 6-4 to continue his comeback after almost two years off the main tour following hip and wrist surgery.

At the Nordea Open in Bastad, Casper Ruud suffered a shock early exit after the Norwegian world number nine was beaten 6-3 6-3 by Brazil's Thiago Monteiro in the second round.

Ruud later returned to the clay to partner Rafael Nadal in the doubles event, with the pair saving a match point as they won 6-4 3-6 (12/10) against Theo Arribage and Roman Safiullin to reach the semi-finals.

"When you cannot win singles you have to try to win doubles," Ruud said afterwards. "It was not the best beginning of the day, but a very nice way to finish the day for me."

Nadal, who is hoping to be at his best for the Paris Olympics, will return to singles action against British number two Cameron Norrie on Thursday.

World number four Alexander Zverev - who has been managing a knee injury following a fall at Wimbledon - saw off Dutchman Jesper de Jong 6-2 6-2 in the first round of the Hamburg Open, where he is defending champion.