The 35-year-old Spaniard was beaten 6-4, 1-6, 6-4 in that game, his second only since his loss at Roland Garros.
The Washington tournament is seen as a warm-up for the US Open, which has become the main focus of the world number three this season.
"
The best news is that my foot is better than yesterday,
" commented after his loss Nadal, who suffered Wednesday in his resumption match in tournament against the American Jack Sock he beat in three sets.
"I will continue to take up the challenge that faces me and I have a chance next week in Toronto"
Rafael Nadal
The Spanish player missed the Wimbledon tournament as well as the Tokyo Olympics after losing to Novak Djokovic in the semi-final at Roland Garros.
"
I will continue to meet the challenge that faces me and I have a chance next week in Toronto
" in Canada, noted Nadal.
The matches played in Washington were intended to prepare him for the US Open which is due to start on August 30. Djokovic, Nadal and Roger Federer will come to this tournament with a record of twenty Grand Slam titles each in their purse, Djokovic with the additional hope of winning all those of the year for the first time since Rod Laver in 1969 Harris, 24, who beat last US Open winner Dominic Thiem in Dubai, is due to meet Japan's Kei Nishikori in the quarterfinals on Friday.