Del Potro overcame a scary late fall to beat Grega Zemlja of Slovenia 7-5, 7-6 (3), 6-0 on Saturday, but said he twisted his ankle and hyper-extended his knee.
The Bulgarian 29th seed, watched by girlfriend Maria Sharapova, staged a temporary sit-down protest to register his anger at court conditions during his 3-6, 7-6 (7/4), 3-6, 6-4, 11-9 defeat to Slovenia's Grega Zemlja in the 2nd Round.