"It's tough to say because I don't feel that great just from the match," the 33-yr-old American said after his 2-6, 6-3, 1-6, 7-5, 6-3 loss to Feliciano Lopez.
Fish, a wild card who plans to retire after the US Open, next faces second-ranked Andy Murray. It was Fish's first singles victory on tour since Aug. 18, 2013, against Evgeny Donskoy at Winston-Salem.
Dimitrov's shoulder injury forced the duo to withdraw shortly before they were supposed to play Rohan Bopanna and Florin Mergea in the quarter-finals on Thursday night.
Pospisil, the No. 2 seed from Canada, beat Taiwan's Lu 6-4, 6-7 (3), 6-4.
The first stop of Mardy Fish's farewell tour ended quickly with a 6-4, 6-4 first-round loss to Israel's Dudi Sela in the Atlanta Open on Tuesday.
Even retirement couldn't stop Andy Roddick from one last chance to play doubles with his friend Mardy Fish in this week's Atlanta Open.
Roddick, who retired in 2012, and Fish will play doubles together next month at the Atlanta Open, where Fish is also going to attempt his latest comeback in singles.
Fish battled for 2 hours, 36 minutes before losing to Harrison, 6-4, 4-6, 7-6 (3) in the first round of the BNP Paribas Open.
Fish has been trying to make a comeback on tour, having missed months at a time over the past two seasons after dealing with heart issues.
The American suffered a heat stroke which gave No. 11 seed Jarkko Nieminen a place in the third round.
Fish - a wild-card entrant - advances to face Jarkko Nieminen of Finland, the tournament's 11th seed, in the second round.
It was only Fish's fourth match since the US Open in September where he withdrew in the fourth round due to illness.
Seventh-ranked Juan Martin del Potro of Argentina has pulled out of the upcoming French Open because of a respiratory virus
Tsonga withstood a fast start by Mardy Fish in the second set to beat the American 7-6, 7-6 in a closely contested third-round match at the BNP Paribas Open.
The 31-year-old beat Bobby Reynolds to book his place in the third round of the BNP Paribas Open.
Djokovic will team with his idol Sampras to play Bryan brothers in a doubles exhibition on March 4.
Serena was ruthless in her 6-0, 6-0 win over Hlavackova, while Federer got a walkover after Mardy Fish withdrew.
Mardy Fish pulled out of his fourth-round match at the US Open because of "health reasons".
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