In his earliest loss at a Grand Slam tournament since 2008, Djokovic lost 7-6 (8), 5-7, 2-6, 7-6 (5), 6-4 on Thursday to 117th-ranked Denis Istomin, a 30-year-old wild-card entry from Uzbekistan. Serena Williams, who — like Djokovic — was ranked No. 2 and is a six-time Australian Open champion, kept her chase for an Open-era record 23rd major title on track with a 6-3, 6-4 win over Lucie Safarova. After winning a rematch of the 2015 French Open final against Safarova, Williams will play fellow American Nicole Gibbs in the third round. “It’s never easy to play in the second round someone you’ve seen in the final of the biggest stage in tennis,” Williams said. Third-ranked Agnieszka Radwanska didn’t last much longer than Djokovic, losing her second-round match to Mirjana Lucic-Baroni 6-3, 6-2. Radwanska reached the semifinals in Melbourne last year; Lucic-Baroni hadn’t won an Australian Open match since 1998 until her first-round win this week. Among those advancing were U.S.